All means of public transport in Berlin (S-Bahn, U-Bahn, tram, bus) can be used with tickets from the BVG (Berliner Verkehrsbetrieb).
The BVG provide different tickets for different zones:

• Zone AB: Berlin city centre
• Zone ABC: Berlin city centre + suburbs
• A “Kurzstrecke” (short distance) costs € 1.70 and is valid for three stations (including changes) with the U- and S-Bahn, or six stations with the Tram (changes not allowed).
• An “einfache Fahrt” (single trip) costs € 2.80 (AB) or € 3.40 (ABC).
• A “4-Fahrten-Karte” (4-trip ticket) costs €5.60 (Kurzstrecke) or € 9.00 (Einfache Fahrt, AB) and entitles you to four trips on any day you choose.
• “Einzelfahrscheine” (individual tickets) entitle you to a trip in one direction. After purchase and validation they are valid for 120 minutes. Within this time you can change as often as you like or interrupt your journey. You may not, however, change your direction of travel, or go in a circle.
• A “Tageskarte” (day ticket) costs € 7.00 (AB) or € 7.70 (ABC). With a Tageskarte you can travel as often as you like during the day printed on the ticket when you validate it and up to 3am on the following day.

All tickets must be validated before beginning your trip and are only valid after this point.
If you intend to use public transport frequently, it is advisable to buy a period ticket (Abonnement). These are called “VBB-Umweltkarten” and are available in two varieties:

• (1) as a monthly card (“Monatskarte”) valid for a particular calendar month:
  These are valid from midnight on the final day of the previous month to midnight on the final day of the month in question.
• (2) as a monthly card valid from a particular day (sliding):
  These are valid from midnight on the first day of validity to midnight on the last day of validity (both printed on the ticket).

During the week, the U- and S-Bahn operate until around 1:30 am, depending on the station. After 1:30 am they are replaced by night busses (a “Nachtbus”). On Friday and Saturday, as well as on official public holidays, the train service runs all night. Trams always run all night.