Roller derby is a contact sport played by two teams of fifteen members roller skating counter-clockwise around a track. Roller derby is played by approximately 1,250 amateur leagues worldwide.
Game play consists of a series of short scrimmages (jams) in which both teams designate a jammer (who wears a unique designation on the helmet; currently a star) and four blockers. The jammer scores points by lapping members of the opposing team. The teams attempt to hinder the opposing jammer while assisting their own jammer—in effect, playing both offense and defense simultaneously.
Source: Roller Derby Wikipedia
The game is played under the Womens Flat Track Roller Derby Association (WFTDA) ruleset, who have some handy videos and infographics like the ones below that help explain the sport.