Contra dancing is a form of social folk dancing that has its origins in New England.  It has roots in English country dance, Scottish country dance, and some French dance styles.

As part of its nature as a social dance, one can attended a contra dance without a partner. 

Contra dances are typically structured such that new dancer can join for an entire evening of dancing with no previous experience.  The figures are taught by a caller.  The music is performed by a live band.

The music played in contra dancing can include Irish, Scottish, English, French-Canadian, and old-time folk tunes.  Popular instruments include the fiddle, the guitar, the accordion, and the piano.

You can read more about contra dancing in general at Wikipedia.