It's the middle of a class period and two hundred students aren't listening to the instructor. Instead, they're engaged in over fifty simultaneous conversations with their neighbors. This probably sounds like a disaster to many teachers. But it's actually a rousing success: the students are discussing a question which challenges them to think about the material and justify their reasoning to their classmates.
Peer Instruction actively engages the students in their own learning. Carefully chosen questions (ConcepTests give students the opportunity to discover and correct their misunderstandings of the material, and, in the process, learn the key ideas of physics from one another.