"Cooperative learning is a successful teaching strategy in which small teams, each with students of different levels of ability, use a variety of learning activities to improve their understanding of a subject. Each member of a team is responsible not only for learning what is taught but also for helping teammates learn, thus creating an atmosphere of achievement. Students work through the assignment until all group members successfully understand and complete it."
see http://edtech.kennesaw.edu/intech/cooperativelearning.htm

"Cooperative learning is an approach to organizing classroom activities into academic and social learning experiences. Students must work in groups to complete the two sets of tasks collectively. Everyone succeeds when the group succeeds."
see http://en.wikipedia.org/wiki/Cooperative_learning

"Cooperative learning is an instructional strategy that simultaneously addresses academic and social skill learning by students. It is a well-researched instructional strategy and has been reported to be highly successful in the classroom."
see http://olc.spsd.sk.ca/DE/PD/instr/strats/coop/
Here you can also find other instructional strategies.