I could try to explain contra dancing but It would be much better to go and try it out! Think line dancing but with groups of 4 and instead lots of swinging! Its live folk music and they teach you the steps to each song/dance slowly before actually starting. Its very easy to learn, loads of fun, and everyone there is very friendly (they'll help you out if you get confused). Its also pretty casual, no big dress up but girls usually like wearing skirts to spin in. :)
Cost is $10 (on some nights students can get in for $8)
There's sometimes a beginner lesson (or at least some easier dances) that runs 8-8:30 when its not as crowded so it's good to come at 8:00, many people usually arrive 8:30 to 8:45.