DICE has documented the five Battlefield 3 multiplayer modes. They are Team Deathmatch, Squad Deathmatch, Conquest, Rush and Squad Rush.
Each of the five modes can be played on each of the nine Battlefield 3 maps available at launch.
Team Deathmatch supports two teams of 12 players (24 in total). The goal is to reach a kill-count before the other team. There are no vehicles in Team Deathmatch.
Squad Deathmatch is for four teams of four (16 in total). Again, the goal is to reach a kill-count before the other squads. An Infantry Fighting Vehicle can be used. An Engineer can repair it.
Conquest for 24 players on console and 32/64 on PC. This is the famous all-out BF warfare mode. Big maps, all the vehicles. The idea is to capture and hold flags. Doing so drains the opposing team's tickets. The first team to run out of tickets loses.