We're often getting asked what Roller derby is, so here's a brief overview:  

Roller derby is played by two teams of fifteen players, who both field up to five members for each two minute jam, simultaneously skating counter clockwise on an oval flat track. Each team designates a scoring player know as the jammer; the other four members are blockers. One blocker can be designated as a pivot which is a special blocker who is allowed to become a jammer during the course of play. 

The bout is played in two periods of 30 minutes. Point scoring occurs during jams that last up to two minutes. During a jam, points are scored when a jammer on a scoring pass (every pass a jammer makes through the pack after the initial pass) laps members of the opposing team.  Each team's blockers use body contact, changing positions, and other tactics to assist its jammer to score while hindering the opposing team's jammer. Certain types of blocks and other play are penalties which the referees will call and the skater will be required to serve time in the penalty box.

For more information check out Roller Derby 101 from the WFTDA.