Roller Derby is a full-contact, female team sport played on a flat oval track. It consists of two teams, each with 5 players on the court during play. A game of roller derby (called a 'bout') is broken into two 30 minute periods containing an unlimited number of 'jams'. Each jam can be up to two minutes duration.
During a jam, one girl from each team is a jammer (identified by the star on her helmet) and the other four girls are blockers. It is the jammer's job to get through the pack of blockers. The first jammer through the pack is the lead jammer and she has the right to stop the jam before the two minutes have elapsed (the decision to do so is strategic). Jammers then score a point for every member of the opposing team they pass from the second lap onwards.
Blockers do their best to stop the opposing jammer - but also have to help their own jammer move through the pack by doing what is known as whipping (ie. pulling or pushing the jammers). The jam is over either when it is called off by the lead jammer, or when the set period of time is reached. The team with the highest points at the end of the bout wins.