Let's Get Postmodern
Improvisational comedy is generally unscripted and free flowing comedy (as opposed to sketch comedy and stand up) and generally involves two or more people. Most improv groups perform on stage. I have a lot of friends who did this in college, taking suggestions from the audience to create funny scenes.
Improv Everywhere is a different sort of outfit. Based in New York City, this group takes comedy to the streets for surreal and wacky public acts. They are probably best known for their Pants Free subway ride, in which a couple hundred people will nonchalantly ride a specific subway train - but none of them are wearing pants. They read the newspaper and get on and off at different stops along the way. No one acknowledges each other and the "real" riders are left to ponder the meaning of it all. It's a highly orchestrated affair, say my friend who have participated.
Or, Best Buy is an electronics store, and all its employees wear blue polo shirts. Improv Everywhere rushed one store with dozens of people all wearing the same shirts and "helping" customers.
One of the guys who used to work at my office quit to do comedy full time (good for him!) and this is one project that he is involved in as well.