“Swirly awesomeness!” Contra dancing is a community event. People of all ages & skill levels dance traditional American social “set” dances to live music. You don’t need to know what to do or to bring a partner. A “caller” teaches the dances, and we change partners every dance. In one evening, you may dance with half a dozen partners, from teens to retirees. Most couples are “opposite sex,” but same gender couples are routine (especially when there are more of one sex that evening, or when there are experienced dancers who’ve enjoyed the new challenge of learning both roles).
In a contra dance, you and a partner dance a series of figures with another couple. (It’s mostly a brisk walking step in circles, stars, etc.). Live music, often traditional tunes from New England, England, Scotland, etc., set the pace and help everyone do the same thing at the same time, so we’re all moving in unison! In about a minute, you complete the series of figures, then you and your partner repeat the same moves with another couple in a long line of dancers, over and over, until the music ends and it’s time to change partners and line up for another dance! An evening includes traditionally ends with a couples dance like a waltz.
Contra dancing is easy to learn, the music is lively, and the dancing can be vigorous, but it is low impact. (It’s good aerobic exercise, always drug and alcohol free.) You interact with your current partner, and everyone else in the set, as you dance with easy steps to live music. Here are some longer explanations: Wikipedia, Gary’s info.