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.
Check out this video for a great a visual explanation.
WFTDA (Women's Flat Track Derby Association) official rules available here
Roller derby is a contact sport played by two teams of five members roller skating in the same direction around a track. Game play consists of a series of short matchups ("jams") in which both teams designate a scoring player (the "jammer") who scores points by lapping members of the opposing team. The teams attempt to assist their own jammer while hindering the opposing jammer — in effect, playing both offense and defense simultaneously. Roller derby is played by more than 1,200 amateur leagues on every inhabited continent.
While the sport has its origins in the banked-track roller skating marathons of the 1930s, Leo Seltzer and Damon Runyon are credited with the basic evolution of the sport to its initial competitive form. Professional roller derby quickly became popular; in 1940 more than 5 million spectators watched in about 50 US cities. In the ensuing decades, however, it predominantly became a form of sports entertainment where the theatrical elements overshadowed the athleticism. This gratuitous showmanship largely ended with the sport's contemporary grassroots revival in the first decade of the 21st century. Although some sports entertainment qualities such as player pseudonyms and colorful uniforms were retained, scripted bouts with predetermined winners were abandoned.
In 2000, Daniel Eduardo "Devil Dan" Policarpo, then an Austin, Texas musician, recruited women to skate in what he envisioned would be a raucous, rockabilly, circus-like roller derby spectacle. After an organizational meeting and a disputed fundraiser, Policarpo and the women parted ways.The women then self-organized as Bad Girl Good Woman Productions (BGGW) in 2001, creating a new generation of roller derby, open to women only. Founders formed four teams, and staged their first public match in Austin in mid-2002. Shortly after, the league split over business plans: The Texas Rollergirls embraced flat-track play, while the BGGW league took the name TXRD Lonestar Rollergirls and went on to skate banked-track roller derby.
The revival then began in earnest, with over 50 similar all-female leagues in existence by late 2005, more than 80 by February 2006, and more than 135 by mid-August 2006. Leagues outside the U.S. began forming in 2006, and international competition soon followed. By mid-2009, there were 425 amateur leagues,including 79 in Canada, Australia, the United Kingdom, New Zealand, Germany, Belgium and Sweden combined.