Swing - the only true American folk dance.
You know the names:
Lindy Hop, Jitterbug, Boogie Woogie, Shag, East Coast Swing, West Coast Swing, Jive.
Swing originated in Harlem in the 1920's, spread across the USA in the 30's, was taken to Europe in the 40's by the GI's, and is still being danced today in night clubs and dance halls all around the world.
Originally, swing referred to a particular style of playing music, and later it became synonomous with the dances people did to swing music. But nowdays, swing type dancing is sometimes done to non swing music and may not even look like swing. So when is it swing? I don't know for sure, but maybe the links below may shed some light on it. And then again, maybe not.
Different styles of swing dance from the US Swing Dance Server
Raper's Swing Dance Corner
Swing at Cornell University (check out the Dance Writings pages)
West Coast Swing Story (California style)
FAQ on Lead and Follow Skills (not styles, but useful anyway)