Stand-up is written. Sketch is rehearsed. Improv is made up on the spot.
The Golden Rule: "Yes, And..."
The core philosophy of clean improv is "Yes, And." * Yes: Accept whatever your scene partner establishes. (If they say "Nice hat," you have a hat. You cannot say "I'm not wearing a hat.") * And: Add new information. ("Yes, I bought it to hide my third eye.")
This prevents the scene from stalling (blocking) and forces collaboration.
Who's Line Is It Anyway?
The most famous example is Whose Line Is It Anyway?, where comedians play games like "Scenes from a Hat." It proved that watching the process of creating a joke is often funnier than the joke itself.
Business Applications
Improv is now huge in the corporate world. The skills of active listening, acceptance, and building on ideas are the exact skills needed for brainstorming and team management.