Simply speaking, group therapy is similar to individual therapy, but with more people. A group therapy program, however, is focused on a specific issue rather than trying to treat multiple issues for each individual.
For example, a group therapy program could focus on treatment for social anxiety (or self-esteem, anger issues, stress management, depression, etc.) and everyone in the group would have similar concerns. Therefore, the treatment can focus on that one particular issue for all the group members.