Long-form improv is the art of creating an entire spontaneous theatrical performance on the fly, usually based on a single audience suggestion. It encompasses a wide variety of performance styles, but is usually funny, often moving, and always one-of-a-kind and immediate.
Nothing is scripted — the improvisors agree on a “form” which are the mechanics and style of a performance piece. The form may be as simple as ‘we are going to do a series of unrelated scenes, and run across the front of the stage to indicate we want to start a new scene’ or as complicated as an improvised horror movie or a Deconstruction.