There are 32 teams in the 2014 FIFA World Cup in Brazil. In order to get into the tournament, teams had to participate in regional preliminary qualifying tournaments. Only Brazil, as the host country, received an automatic bid into the final tournament. (Source)
These 32 teams are organized into 8 groups of 4, labelled A through H. To decide which teams are in each group, first the top eight teams are seeded into the first position of each group. (Brazil, as the host country, is automatically seeded as team A1.) After these 8 teams are each put into separate groups, the rest of the teams are placed into the groups based on geography and random draw. (Source)
The first stage of the tournament is called the Group Stage. In this stage, each team plays the other three teams in their group. That means that each of the 32 teams plays 3 matches, for a total of 6 matches in each group, and 48 matches in the entire group stage. The Group Stage is scheduled for June 12-26, 2014.
After the Group Stage is done, the teams in each group are ranked by their performance in their three matches, and the top two teams in each group go to the next stage of the tournament.
These remaining 16 teams go into the Second Stage (or Knockout Stage), a single-elimination tournament. The Second Stage begins on June 28, and the final match is scheduled for July 13, 19:00 UTC, in Rio De Janeiro.