A concentration camp was established by the Nazis in the suburbs of the Polish city of Oswiecim which - like the rest of Poland - was occupied by the Germans from the beginning of the Second World War (1939-1945). The name of the city of Oswiecim was changed ('Germanized') to Auschwitz, which became the name of the camp as well.
The camp was continually expanded over the next 5 years and ultimately consisted of 3 main parts: Auschwitz I, Auschwitz II-Birkenau, and Auschwitz III-Monowitz. Auschwitz also had over 40 sub-camps in the neighboring cities and in the surrounding area. Initially, only Poles and Jews were imprisoned and died in the camp. Subsequently, Soviet prisoners of war ('POWs'), gypsies, and prisoners of other nationalities and minorities were also incarcerated there.
From 1942 onwards, the camp became the site of one of the greatest mass murders in the history of humanity, committed against the European Jews as part of Hitler's plan for the complete destruction of that people ('the Final Solution'). The vast majority of the Jewish men, women and children deported from their homes all over occupied Europe to Auschwitz were sent immediately to their deaths in the Birkenau gas chambers upon arrival, usually trained in in overcrowded cattle wagons. Their bodies were afterwards cremated in industrial furnaces in the crematoria.
At the end of the war, in an effort to remove the traces of the crimes they had committed, the SS began dismantling and razing the gas chambers, crematoria, and other buildings, as well as burning documents. Prisoners capable of marching were evacuated into the depths of the German Reich. Those who remained behind in the camp were liberated by Red Army soldiers on 27 January 1945.
A 2 July 1947 Act of the post-war Polish Parliament established the Auschwitz-Birkenau State Museum on the grounds of the two extant parts of the camp, Auschwitz I and Auschwitz II-Birkenau.
The site was added to the UNESCO World Heritage List in 1979.
There are quite frequent and inexpensive buses (13zl each way online or 14zl in person - Lajkonik) or minibuses (8zl each way - depart from the basement level of the main bus station) to and from the main bus station in Krakow, or guided tours are available from most hotels or tourist information centres. The bus takes about one and a half hours - it is usually busy and stops locally along the way. The mini buses run every 20 minutes during the morning hours. They return from Auschwitz at 13:16, 15.36, &16.01 as of October 2012. A larger bus also leaves from Auschwitz at 14:20, 15:15, 16:00, 16:30, 17:00, 17:30, 18:30 and 19:30 (again as of March 26th, 2015). The larger bus leaves from the museum area, while the mini bus leaves from across the street from where you are let off. The minibus can be very crowded and won't manage to fit everyone at the busstop. There is a bus connection with Katowice city too, the buses leave Katowice from Plac Korfantego and arrive at the Oswiecim bus stop, from which one can catch a bus to Auschwitz. the ride takes and hour. Also, trains regularly run from Krakow to Oswiecim, you can purchase a one way ticket for 9,5 zl.
There is a shuttle bus between Auschwitz and Birkenau. It is free and goes every half hour (from Auschwitz to Birkenau it leaves on the hour at half hourly intervals and going the opposite way it is 15 minutes of the hour at half hourly intervals - please check the timetable at the bus stop as the intervals and starting hour of the free bus operation may change depending on the season), or you can just walk the two miles between the camps (although it isn't a very nice walk as it is along the roads). If you've just missed a bus, a taxi between the sites will cost about 15 zl.
Local (Krakow - 1hr30m, Katowice - 1hr20m) trains and one daily international nighttrain (running between Krakow and Prague with direct carriages to/from Vienna and Budapest) stop at the nearby Oświęcim railway station. A bus can then be caught to Auschwitz I (there is a bus stop in front of the railway station), or you can walk there (approx 1.5 km) in about 20-25 minutes (take Dworcowa street and then Obozowa street - there are signs along these streets directing you up to Auschwitz I).
Tours from Krakow
Several companies provide tours from Krakow for around 130 zł. They advertise heavily so you'll have no problem finding one. These tours involve a minibus pick-up from anywhere in Krakow, and a few hours guided tour.
Entrance is free, without a ticket, only after 3 p.m. Be aware that because of the large numbers of visitors entry to the Auschwitz I site is exclusively on a guided (and unfortunately rather rushed), group basis from 10:00 a.m. to 3:00 p.m. during the period from April 1 to October 31. You can visit the site on your own (highly recommended because you can go at your own pace, see what you want to see and have a much more meaningful experience) if you arrive before 10:00 a.m., this is possible if you're staying in Krakow and don't have your own car with some trains from Krakow Glowny arriving between 8am-10am. Guided tours cost 40zl (discounted price for students up to 24 years of age is 30zl). Students with an ISIC card are granted free entrance during tour hours (as of May'14). If you're a small group (~4 or less) it's not too hard to buy tickets on site (you might have to wait a while depending on availability), but larger groups should book tickets in advance.
The tour takes 3 hours with a 20-minute break after about 1.5 hours. Tours run every 15 minutes or every 30 minutes depending on language.
The Auschwitz II-Birkenau site is open for visitors without the guide during the opening hours of the Memorial. You can also book a private tour guide from the Auschwitz Museum  site for a 6 hour study tour (400zl).
The Museum is open all year long, seven days a week, except January 1, December 25, and Easter Sunday. The Museum is open during the following hours:
•8:00 AM - 3:00 PM December through February •8:00 AM - 4:00 PM March, November •8:00 AM - 5:00 PM April, October •8:00 AM - 6:00 PM May, September •8:00 AM - 7:00 PM June, July, August
These are the hours for visiting the site of the camp. The office of the Former Prisoners' Information Section, Archives, Collections, Library, Administration, and other departments are open from 8:00 AM-2:00 PM, Monday through Friday (except holidays).
The 12:30 tour will get you back for the 4pm bus back to Krakow (goes from behind Auschwitz I, not the main road).
The Auschwitz Memorial and Museum is easily navigated on foot. There is a free bus between Auschwitz and Birkenau.
Tours are provided by the museum for a fee in various languages, and are recommended if you want a deeper understanding of the site, but they are unfortunately somewhat rushed, and you can get a pretty good feel by buying a guidebook and map (small, simple guide = 5zl; more detailed "souvenir" guide = 12zl - covering Auschwitz and Birkenau) and wandering around on your own (provided you arrive before 10:00 a.m. - after 10:00 a.m., for crowd control purposes, guided tours are mandatory). Each exhibit is described in Polish with other language translations. The scope of the evil and terror that occurred here is almost unimaginable and a guide can help to put in context what a room full of human hair or what a thousand pairs of infant shoes means. They'll also tell you about former prisoners who have returned to see the museum.
There's a basic cafe and cafeteria in the main visitors' centre of Auschwitz 1 and a coffee machine in the bookshop at Birkenau. More options are in a commercial complex across the street from Auschwitz 1, although the quality of one (the Art Hamburger) is rather poor, but a cheap and quick eat. There are hot dog stalls and similar outlets outside the main museum at the end of the bus/car park.
You cannot sleep at the camps. The closest accommodation options are in Oświęcim.
Please remember that you are essentially visiting a mass grave site, as well as a site that has an almost incalculable meaning to a significant portion of the world's population. There are still many men and women alive who survived their time here, and many more who had loved ones who were murdered or worked to death there, Jews and non-Jews alike. Please treat the site with the dignity, solemnity and respect it deserves. Do not make jokes about the Holocaust or Nazis. Do not deface the site by marking or scratching graffiti into structures. Pictures are permitted in outdoor areas, but remember this is a memorial rather than a tourist attraction, and there will undoubtedly be visitors who have a personal connection with the camps, so be discreet with cameras.