Potsdam is the capital of Brandenburg and borders Berlin. The town has population of approx. 159,000. It is widely known for its castles and landscape as a World Heritage Site. Potsdam is more than 1000 years old. Many historic buildings are under re-construction after World War II and the period of the GDR.
For most of its recent history Potsdam has not been accessible from Berlin. The last station before the former GDR was Wannsee. Many of the buildings that are visible today have been reconstructed after the bombings of the Second World War and after the lax care of the East German Government. The city as we see it today is the work of five architects (After the Great Elector said: "Das ganze Eyland muß ein Paradies werden" (The whole island must become a paradise)): Peter Joseph Lenné, Georg Wenzeslaus von Knobelsdorff, Carl Phillipp Christian von Gontard, Karl Friedrich Schinkel and Friedrich Ludwig Persius, and of course King Frederick the Great and Emperor Wilhelm II.
It is important to realize that most of the tourist attractions in the city are UNESCO World Heritage sites - these are:
Sanssouci Park and the Crown estate of Bornstedt
The New Garden (including the Pfingstberg and the Russian colony of Alxandrowka)
Potsdam is in the Berlin metropolitan area and served by both inner and outer suburban trains, called S-Bahn and Regionalexpress respectively. Several lines serve Potsdam, but not the same parts.
For central Potsdam, you should travel to Potsdam Hauptbahnhof, which is the terminus for S-Bahn S1 (and then S7 from Berlin-Wannsee) and also served by Regionalexpress RE1 towards Brandenburg or Magdeburg. From the most centrally located station, Friedrichstraße, it takes 47 min to Potsdam Hbf by S7 (40 min: S7+S1) and 28 min by RE1, the latter making limited stops in inner areas.
For the Schloss Sanssouci, Park Sanssouci and Krongut Bornstedt, you should take an RE1 train direction Brandenburg (not Magdeburg) and get off at Charlottenhof, the station following Potsdam Hbf. From there the mentioned attractions are 10 minutes away on foot.
For Babelsberg, you should take the S-Bahn S7 to Babelsberg, the station preceding Potsdam Hbf.
For the Medienstadt, you should take an RE7 train direction Bad Belzig or Dessau and get off at Medienstadt Babelsberg.
Your ticket must be valid for at least Berlin zones ABC in order to travel to Potsdam; however, tickets and travelcards valid for the entire metropolitan area are of course also valid. For more information on this, visit the local transport website of the Berlin metropolitan area 
If you are visiting Potsdam while staying in Berlin, you could take a boat to go there. At Berlins lake Wannsee, close to the S-train-station "Wannsee", several boats depart for Potsdam every day. Shipping companies include "Stern und Kreisschiffahrt" and "Reederverband Berlin", a roof organisation of several smaller companies. The boattrip from Berlin-Wannsee to Potsdam lasts around 1hr.
Berlin has two airports which are not far away from Potsdam. From Tegel bus X9 connects to Jungfernheide S-Bahn station, from where you can take the S42 to Westkreuz and then the S7 to Potsdam (for passengers to Charlottenhof or Medienstadt RE trains connect to the S7 at Wannsee). From Schönefeld, suburban train line RB22 connects to Potsdam Hbf via Charlottenhof (passengers for Medienstadt should change to RE7 or MR33 at Michendorf, while passengers for Babelsberg should change to S-Bahn S7 at Potsdam Hbf).
The tram and bus system in Potsdam is very modern and overall excellent though confusing. Potsdam is, together with Berlin and parts of Brandenburg, part of the VBB transportation network. That enables you to change trams and buses pretty much at will, as long as your ticket is valid for the zone you're in. The inner city of Potsdam is in Zone A, the outer parts are in Zone B, and the hinterland is Zone C. Be aware though, as Potsdam itself lies in Berlin's Zone C - Berlin's outer parts are Berlin Zone B and so on. A Berlin ABC or BC ticket will therefore cover your journey within Potsdam.
Tickets can be bought at ticket machines in every tram and bus. Most machines of newer type accept bills, coins, rechargeable "Geldkarten" cards and EC direct-debit cards; though some of these options may be temporarily unavailable and it's advisable to keep some coins at hand. There is an experimental ticket-less system  which allows you to pay automatically by tracking your journey via a smartphone and is worth checking out if you meet the requirements. It's best to get detailed information about prices and zones at Potsdam's central station or on the VBB website. The transport, although confusing at first, is pretty logically laid out - each tourist attraction has its own bus / tram stop (with the appropriate name) and the staff of the VBB are extremely helpful, with most speaking English very well.
For tourists there are five main lines in and around Potsdam:
Schlösser-Linie (Palaces Line):
Bus No 695 from Potsdam Hauptbahnhof - Bahnhof Pirschheide
With stops for: The Historic Steam Engine House, Sanssouci Palace and Park, The Orangery, the Dragon House, the Belvedere, The New Palace and a path to the Charlottenhof Park
Krongut-Linie (Crown Estate Line):
Tram No 92 from Potsdam Hauptbahnhof - Bornstedt Kirschallee
With stops for: The Dutch Quater, The Russian Colony (Alexandrowka), the Pfingstberg, the Potsdam Biosphere and the Volkspark
Tropen-Linie (Tropics Line):
From Potsdam Haupbahnhof - Viereckremise
With stops for: The Historic City Centre, the Russian Colony (Alexandrowka),the Pfingstberg, the Potsdam Biosphere and the Volkspark
Cecilienhof-Linie (Cecilienhof Line):
From Potsdam Haupbahanhof take Tram Nos 90/92 to Reiterweg / Alleestraße, then Bus No 692 - Höhenstraße
With stops for: The Russian colony (Alexandrowka), the New Garden, the Marble Palace, Celcilienhof Palace and the Pfingstberg
Filmstadt-Linie (Film City Line):
From Potsdam Hauptbahnhof - Stern-Center / Gerlachstraße
With stops for: Babelsberg palace and park, Flatow Tower, the Arcaded Court House, the Neubabelsberg Villa Colony, Babelsberg Media City and the Film Park.
Park Sanssouci - This immense park outside Berlin in the city of Potsdam is on the UNESCO World Heritage list, along with all its outbuildings. Get there early: there is a 2000 person/day entry limit at Schloss (Castle) Sanssouci, a fabulous rococo palace with amazing interiors. One of the most noteworthy rooms in the palace is the Konzertsaal (concert hall) - dripping in rococo glory. (Official information)
The New Garden hosts the Marmorpalais and the Palace Cecilienhof.
Gedenk- und Begegnungsstätte Ehemaliges KGB-Gefängnis Potsdam, Leistikowstr. 1 . Literally the Memorial and Meeting-place Former KGB Prison Potsdam. From August 1945 it was occupied by soviet forces who oversaw its reconstruction as a prison for the counter-intelligence services. Today it's been left standing to remind people of the depressing reality of dictatorships. Open from May to October, every Saturday and Sunday 11-17h.
Holländisches Viertel - Right in the middle of Potsdam are the "Dutch Quarters", built in the 18th century by Dutch master builder Johann Boumann to attract other Dutch artisans. The quarters consist of 134 red brick houses, divided into four blocks. The history of the Dutch Quarters can be seen in the house in Mittelstraße 8.
Alter Markt, the old market is the historic city center, the Stadtschloss (city castle) is under re-construction. You can visit the Nikolaikirche there.
New Market, the new market is an original preserved market square from the 18th century.
Park Babelsberg is a beautiful park with a gothic-style castle. It is also part of the World Heritage site. One part of the university is placed here, so expect some students.
The Russian Colony - Alexandrowka. Alexandrowka consisits of a few "Russian Style" houses built for Russian immigrants during the reign of "The Soldier King". House No. 2 (Haus 2) has a very interesting museum while House No. 4 ( Haus 4) has an excellent café. Visit their website for further information .
Krongut Bornstedt - an architectonical complex in Italian style with a palace and a garden
Potsdam has several interesting museums to offer:
Haus der Brandenburgisch-Preußischen Geschichte, . A museum about Prussia's history.
Museum of Natural History
Film Museum , exhibitions about cinema history in Potsdam and movie theatre.
Filmpark Babelsberg movie theme park. Well worth a visit, the first German expressionist film in the world was made at these studios. The next-door studios are home to Gute Zeiten, Schlechte Zeiten, a famous German soap opera.
Potsdam Museum at the Dutch District.
Altes Rathaus Changing exhibitions at the old town hall.
Gemäldegaleria. The beautiful collection of paintings at the Sans Souci Palace.
The Glienicke bridge (Glienicker Brücke). It is a bridge in Berlin which spans the Havel River to connect the cities of Potsdam and Berlin near Klein Glienicke. It was completed in 1907edit
Visit Sanssouci Castle - the baroque decoration is wonderful.
Take a cycling tour of the city
Walk around Sanssouci Park, with its many castles, follies, cascading terraces, mini-gardens, and romantic ruins.
Climb the Babelsberg
Stroll around New Garden (part of the UNESCO world heritage ensemble), on the banks of the river Havel. It was here that Churchill, Stalin and Truman met for the Potsdam Conference, at the end of World War II.
Visit the Dutch quarter, the Russian Colony, and the Weberviertel in Babelsberg (the 'weaver's quarter', built to accommodate artisans from Bohemia) - three architectural oddities commissioned by the Prussian kings to attract foreign craftsmen to Potsdam.
Just north of the city, in the woods overlooking the river, is a historic Russian church and lodge by the name of Nikolskoe. Now a popular cafe, it makes for a good pit stop on the way to nearby Peacock Island.
Walk around the picturesque south bank of lake Griebnitz, and into the streets of Neubabelsberg - a typical German 'Villenkolonie' of the Belle Epoque. Many of the grand homes' Jewish occupants were evicted by the Nazis, who then moved in themselves.
Visit the 'Studio Babelsberg'  for a 'Studio Tour' , in the Babelsberg film studios - Germany's Cinecitta. The centre of German celluloid expressionism, Marlene Dietrich, Fritz Lang and so many others worked here in the 1920s; more recently, Quentin Tarantino shot Inglourious Basterds on the premises.
Potsdam's nightlife is somewhat dominated by the nearby Berlin, but there are some good clubs and pubs worth visiting. Interesting pubs are located in the Holländer Viertel around Mittelstrasse, or in the city. For dancing there are the old traditional clubs Waschaus, Lindenpark or Nachtleben
Archiv - alternative youth centre in the Leipziger Strasse - here you find regularly Parties and concerts. Freiland Potsdam It is a big area near the Hauptbahnhof where you find a lot of diffeent activities: concerts, parties, youth clubs, art clubs and a lot of social organisations.
Hafthorn - Famous pub with a really nice Biergarten in summer and it is famous for it's handmade Burger
La Leander - Coffee/Bar for the gay-lesbian-scene, but also hetero guest are very welcome.
Das Café Heider
Waschbar - Szenekneipe in Geschwister-Scholl-Straße 82. Here you have a lot of different beers, menus and you can wash your clothes.
On the Rocks - Rockbar in the Lindenstraße.
Rückholz - Gemütliche Kneipe in the Sellostraße. You can sit on sofa or easy chairs while you have a talk or wathc the art that you find on the walls. Eating is recommended because the hosts have a lot of good ideas and all the dishes are handmade and with good ingredients.
Club Charlotte - Located directly in the city - Charlottenstrasse. Here you find nice concerts and parties. Waschhaus- You find teh Waschhaus in the "Schiffbauergasse" near the Hans-Otto-theatre. Here you find parties, concerts, cinema, art exhibitions etc. Lindenpark - The traditional address in Babelsberg - concerts, parties, skater park and more. http://www.nachtleben-potsdam.com/events/ Nachtleben] - Party music for everyone Palmenzelt - This club is only open in autumn, winter and early spring. The parties take place between a lot of palmes in summer the club organises regularly parties on a ship. Located at the harbour (near main station
Stilbruch - Lounge
Quartier - Potsdam Hostel from 19 Euro (Hostel Pension Backpackers), Ribbeckstraße 41 (14469 Potsdam), ☎ +49 331 2739939, . checkin: 16:00 - 20:00; checkout: 11:00. The Potsdam Hostel is in the middle of the cultural center of the city, but still offers a very relaxing and quiet atmosphere. The nearby area includes many exciting historical sites, from the “Italian Village” to the Sanssouci Park, both of which are in close walking distance from the Hostel: our property is directly on the border of the park. Our neighborhood also offers many interesting attractions; across the street you can sample the rich culinary offerings of the “Krongut Bornstedt” restaurant and later in the evening you can try homemade Büffelchen beers. . For breakfast, the Potsdam Hostel offers a wide range of option everyday from 7:00. Many shops are available nearby, as well as hairdressers and cafes. Our clean, comfortable, gender exclusive shared rooms come with 4 beds and include access to bathrooms and showers. Our community room offers complete kitchen facilities, TV, free wireless access, as well as a PC for those who leave their laptops at home.19 €/bed/night. edit
The Pfaueninsel (Peacock Island) is a small island with free living peacocks.
Werder (Havel) is a small town just west of Potsdam. Every year it is the home of the Baumblutenfest , to celebrate the large amount of fruit trees blooming in spring (especially sour cherry trees, from which they make wine).