Derby is played on a flat, indoor, oval track with two competing teams. Each team fields 5 players at a time- made up of 1 Jammer, 1 Pivot, and 3 Blockers. The largest majority of skaters together on track is called the "pack". The Jammer is identified by a star on their helmet cover.
The Jammers must skate fast and fight through the pack to score points. The first Jammer through is called "Lead Jammer" and has the ability to end the jam when they choose.
Once the Jammer exits the pack, they then race around the track in order to lap the pack. For every member of the opposite team they pass, they score one point.
A jam ends when the two minute time period is over or the lead jammer calls off the jam.
Simple right? Oh - and it's really, really fun!!
Video is from the Roller Derby documentary "Roller Derby Till I Die" which follows members of the London Roller Girls and is a great introduction to the passion and commitment found in the sport.