Well, I missed Marc's spiel the other night, and I don't mean to intend to start a thread, but I do believe most improv folks would accept the fundamental notion expressed below (from wikkipedia):
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Modern improvisational comedy, as it is practiced in the West, falls generally into two categories: shortform and longform.
Shortform improv consists of short scenes usually constructed from a predetermined game, structure, or idea and driven by an audience suggestion. Many shortform games were first created by Viola Spolin based on her training from Neva Boyd[1]. The shortform improv comedy television series Whose Line Is It Anyway? has familiarized American and British viewers with shortform.
Longform improv performers create shows in which scenes are often interrelated by story, characters, or themes. Longform shows may take the form of an existing type of theatre, for example a full-length play or Broadway-style musical such as Spontaneous Broadway . Longform improvisation is especially performed in Chicago and New York City. Perhaps the best-known, and considered the first, longform structure is the Harold, developed by ImprovOlympic cofounder Del Close. Many such longform structures now exist.
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Remember that Johnstone developed his formats not only to entertain audiences much like sports do, but to provide "training" for improvisors (see the Tao of Micetro
http://forum.austinimprov.com/viewtopic.php?t=4320). Which is why I encourage everyone who plays to really take heart of notes, because you will improve over time as you play Micetro. We all will, regardless of experience.