Contra dancing is the original American "country line dance." In a Contra dance, parallel lines of dancers stand opposite their partners. A Caller leads a walk through before the music starts and then prompts the dancers while the music is playing. A dancer and his/her partner form a couple. Over the course of the dance, in response to the Caller's instructions, each couple interacts with the couple next to them to form a four person set and each set interacts with the sets on either side of them. Over the course of a dance each couple moves up and down the hall, interacting with every other couple in their multi-set line.
Contra is danced to music played by live bands - bands may include fiddles, flute, guitar, piano, and bass or other instruments. Traditional jigs, reels, and hornpipes from the Scots-Irish tradition of New England, Quebec, and Appalachia form the basic repertoire.
There is no fancy footwork involved in Contra dancing, but the instructions given by the Caller do form a series of repeating figures that dancers memorize over the course of a dance. As this happens, the Caller provides fewer and fewer prompts until s/he drops out entirely; leaving you, your partner, and the others in your line to finish the dance, accompanied only by the music provided by the band.
Absolutely NO dance experience is needed to Contra dance, you need only walk to the music. No special attire or shoes are needed - wear comfortable shoes and clothing you can be active in.