Here are some long-winded attempts at describing Contra, but it's really easier to just come along and try it yourself:

• What is Contra?
 Created by Gary Shapiro

• An introduction to Contra Dancing
  Created by Urbana Country Dancers

• An explanation of the basic dance moves (pdf file)
Created by Tucson Friends of Traditional Music

• Secrets of Contra Dancing
Created by Greg Allemang

• Wiki Books - Contra Dance Tips and Techniques
  

Guide

• Dance Etiquette Created by Country Dance New York