It's a version of rugby union, but with only severn players per side on a full size pitch. The extra space available makes it a very fast game. Lineout and scrums also (obviously) go with reduced numbers. The game itself consists of two seven minute halves, which is about the most the players can go for without somebody's heart exploding. Other than the reduced numbers and time, there are only two significant changes from standard 15s rules. All kicks at goals (penalties and conversions) must be drop kicks -- you are not allowed to bring a tee onto the field. Also the restarts after a score are reversed, in that it is the team which has just scored which kicks off to restart play.
Its just rugby with seven players on the pitch however on a full sized pitch