What the Boff is Improv?
Improvisation is when you make it all up as you go. No scripts. No props. No sets.
Improv is all about actors getting an audience suggestion, then coming together to create a scene out of basically nothing. We use the suggestion as a kick off point, but it's trust, agreement, and listening etc. that keeps the scene going.
To learn the cool nitty-gritty of improv check out the following books and websites.
TRUTH IN COMEDY - Charna Halpern, Del Close, Kim Johnson
IMPROVISE - Mick Napier
Here's a link to book excerpts on different aspects of improv.
Here is a massive improv forum that can answer a lot of questions.