Improv (from improvisation) is a theatrical form where the plot, characters, and dialogue are made up on the spot, with actors using suggestions taken from the audience.
Improvisational theater is unique in that if you watch one performance, you can be sure that you will never see another one played out exactly the same way. Improv is different every time.
It's spontaneous, energetic, and fun. But like all great things, you need to experience it firsthand, and only after seeing it can you fully appreciate it.
The improvisational form of theater resembles traditional theater, but it does not involve a pre-written script. It consists of actors playing off each other, directing themselves, and simultaneously creating the plot without prior rehearsal.