Contra dancing is a North American folk dance which grew out of English Country dancing during the colonization of North America.  It has continually evolved in the same way other dance movements have and in recent years has seen a huge revival in the USA.
While having similar moves as square dancing, it is characterized by the usual formation of two long opposing lines and the interaction of couples up and down the line.  However, a variety of formations are used and the lines may be straight, or in circles or triangles. Quadrilles (square formation) are also in common use within a contra dance program.
The term ‘contra’ means opposite and in standard contra dances the partners of a dancing couple are opposite each other in the lines.  In the non-standard lines, the partners may be adjacent to each other as they are in a square.  Contras are often called ‘longways’ and could be any length, but some are limited to three, four, five or six couples.  Contra dancing is further characterized by the fact that dancing is performed to the phrasing of the music which varies from jigs, reels, and hornpipes to polkas, two-step and the waltz.  Square dance singing calls are often used and Scottish Country dance music has become very popular.  In many regions throughout the US and in a few Canadian cities, live music is used.
To find out the real answer to the question “what is contra dance”, one must try it.