Over 10 million people visit Calais every year. Many of these visitors are British day-trippers arriving by ferry or on Channel Tunnel shuttle trains: one reason being due to the fact that France has lower taxes on alcohol and tobacco than the UK. Large warehouse supermarkets have sprung up on the outskirts of Calais just to serve these British "booze cruisers", who return to the UK with their car suspensions groaning under the weight of cheap alcohol and cigarettes. Since Belgium now has cheaper tobacco than France, it is becoming increasingly common for British day-trippers to travel to the nearby town of Adinkerke, 60km from Calais, and return to France to purchase their alcohol. While most tourists view the town as simply a port where one can buy cheap alcohol, cigarettes and groceries, there is more than that.
Passengers travelling from the UK to France go through French passport/identity card checks in the UK before embarkation, rather than on arrival in France. Passengers travelling in the other direction go through French exit checks and UK passport/identity card checks in France before embarkation. UK customs checks take place on arrival for those arriving by ferry or Eurostar, and before embarkation for those travelling by Eurotunnel.
Calais is connected to Dover in England by ferry services operated by P&O Ferries  and DFDS Seaways . Ferry crossings between Dover and Calais take typically around 1 hour 30 mins. Ferries operate 24 hours per day - combined between the operators, there are up to 41 sailings daily. The check-in time is 30 minutes before departure for vehicles and 45 minutes before departure for foot passengers.
Bear in mind that foot passengers are not allowed on services operated by DFDS Seaways, and can only travel on P&O services between the hours of 08:45-19:05 between Dover and Calais and 07:05-20:55 between Calais and Dover.
The Strait of Dover is one of the most frequented shipping lanes in the world. The Calais ferry port allows easy access to the town itself as well as the warehouse supermarkets outside the town.
Free car parking facilities are available in front of the Calais ferry terminal and the maximum stay is three days.
Another popular ferry route is Dover to Dunkirk (near Calais) which is operated by DFDS Seaways and takes around 2 hours. This route is popular due to the fact that the nearby town of Adinkerke (in Belgium) has lower taxes on tobacco than both France and the UK, thus British day-trippers have convenient access to both cheap tobacco and alcohol rather than just the latter via this ferry route.
Fares for the Dover-Calais ferry vary heavily, depending on which company you book with and when you travel. As a typical example, if booked online, P&O charge £35 for a single, £28 for a 3-day return and £45 for a 5-day return for a car and up to 9 passengers.
It should be noted that in case of bad weather (or strikes), the ferry traffic is diverted to the tunnel and long queues can form on the autoroute itself.
Note that Calais Fréthun station is some distance from Calais city centre, but there are a number of trains operating between Calais Fréthun and Calais Ville (in the city centre), as well as taxis.
The Eurostar timetable is available from their website .
Those travelling by Eurostar between Brussels and Calais are within the Schengen Area, however passengers are still subject to UK immigration controls. The Brussels Midi-Calais Fréthun route can't be purchased through the Eurostar website, however it is available on the Belgian Railways website .
Eurostar fares start from £66 return (under 26) and £69 return (adults) from the UK to Calais .
Eurotunnel (Le Shuttle)
Running from Folkestone to the western edge of Calais (Coquelles), this car shuttle service operated by Eurotunnel  takes about 35 min (although only about 20 min of the journey is in the tunnel) and offers the fastest way to travel between the UK and Calais. This train service is for passengers with cars only, who remain with their car for the duration of the short trip. There are toilet facilities onboard.
Running from Lille, Boulogne, Dunkirk and Paris, Calais is well supported by the French national rail network . Calais Ville is the station serving the city centre, whilst Calais Fréthun is located on the outskirts of Calais. There are trains that operate between Calais Ville and Calais Fréthun stations.
Train & Ferry
Southeastern  offer the Calais Day Tripper ticket - an integrated same-day return from London to Calais - for around £40, which can be purchased on the day at Charing Cross or Victoria stations. This ticket is significantly cheaper than buying a one-way train ticket to Dover plus a one-way foot passenger ferry fare to Calais. However, the ticket is not valid on any high-speed services to Dover - just the ordinary slow trains.
Note that P&O ceased its shuttle bus between Dover Priory train station and the ferry terminal in 2014. However, they are still linked by the number 63 bus, which departs from outside the railway station.
Calais is directly linked to the French autoroute system. The two autoroutes that serve Calais are the A26/E15 (Autoroute des Anglais), which provides access to the heart of France (including Paris and beyond) and the A16/E40, a gateway to the Benelux and the northern European motorway systems. The A16/E402 (L'Européenne) also directs you to western France, including Boulogne, and is an alternate route to Paris and the South.
Eurolines  operate daily buses from Paris, Lyon and London to Calais. Some services from London also stop at/near Gillingham, Canterbury, Folkestone and/or Dover en route. The Eurolines stop in Calais varies - services which operate across the Channel via the tunnel stop at the Cite Europe shopping centre, whereas the services which use the ferry stop at the ferry terminal. Fares from the UK to Calais are around £33 for a single and £44 for a return.
An alternative option for travellers from the UK is to take a National Express  coach from London to Dover (Ferry Port bus stop) and board the ferry as a foot passenger. This option, however, is significantly more expensive than the Eurolines service.
Most people travel by car around the town, but there is a comprehensive network of buses operated by Vivabus , running from the town centre to the suburbs and Cité Europe. The port is served by a special shuttle bus, although via a footbridge the centre and the harbour terminal are only 15 min apart.
Bus 1 is useful for reaching the Cité Europe shopping mall. Services run every 30 min during the day and costs €1 per ticket, €3 for a day pass or €7.50 for 10 tickets - board through the front door and pay the driver.
There is a free minibus service called Balad’in, which runs in the centre of Calais. The buses are bright yellow/blue and easy to spot. They stop at yellow Balad’in bus stops. The buses operate from 0900 to 1900, Monday to Saturday and operate every 12 minutes. The route goes from Jardin des Plantes to Matelote (near the lighthouse) via the Calais-Ville train station.
Timetables for all buses can be found here .
There are a few different bike rental programs in Calais:
As is the case in the rest of France (and most of Europe), the sole official currency is the euro (EUR, €). However, many shops frequented by tourists (particularly the alcohol-selling warehouse supermarkets on the outskirts of the city) also accept the pound sterling (GBP, £).
For a lot more infos, have a look on the Couchsurfing Wiki Page of Calais. 
It is not unknown for strikes to occur in Calais. Typically, strike action shuts down the port, and sometimes the Channel Tunnel as well. It is therefore advisable to check whether any strikes are due to occur when you plan to travel to Calais.
For a more 'traditional' experience of France, the town of Saint-Omer is a mere 26 miles (43 kilometres) down the road from Calais and is well-signposted. The Saturday morning market on the main square, the Place Foch, can be enjoyable. There are also good affordable restaurants and friendly bars, as well as interesting architecture.
Dunkirk is 30 miles (48 kilometres) east of Calais along the A16.
To hitchhike to Belgium or the Netherlands, if you leave the terminal, you must walk 500 m through a large car park, after which you will see roundabout and an on-ramp to the motorway. That is the place to hitch a ride: there is enough space even for a big lorry.