Contra dances are the American idea of Country Dances. They are popular in America because they are easy enough for beginners to join in, but varied enough to keep experts interested.
Traditionally we have a caller to explain each dance and remind you as you go along, and a live band to keep you on your toes.
Dress - pretty well anything goes. I would avoid high heels; expect to get hot since the band will inspire you to burn up lots of energy.
There is no need to bring a partner with you - since we dance as much with everyone else in the set as our partner generally we change partners between each dance anyway. Don't let that stop you bringing half a dozen friends though!
Try this if you want an American description of contra dance.