Improv is made up on the spot. No scripts, no lines, no teleprompters.
If you've seen "Whose Line is it Anyway?," you've seen improv.
Improv comes in several forms. With long form improv you might see an entire play or story of several scenes connected together by one transition or another.
With short form improvisation, you'll see songs, scenes and soliloquies made up on the spot, usually with some unusual 'rule,' 'game,' or restriction added to it. Here are some quick examples of 'games' or as some improvisers call them, 'hoops:'
Word at a Time Story - 2-4 improvisers in a line generally. Together they tell a seamless story. Each improviser only gets 1 word. The next in line puts the next word on, and so on...
Scene 3-Ways - A short simple 'neutral' scene is improvised. Then the audience suggests 3 unique styles or genres, and the scene is replayed three times, once in each of those styles. The styles might be movie styles, styles or genres of books, or even musical styles.
Forward/Reverse - one of a myriad of 'called' games, where someone randomly calls out directions or cues that force the action to change. In forward/reverse, whenever the caller says "forward," the scene moves forward. When "reverse" is called, the scene moves in reverse from that point until forward is called again.