The game itself is a series of races between two teams of five players, a pivot from each team in front, 3 blockers from each team in the middle, and a jammer from each team in the back.  The group of pivots and blockers comprises the “pack.” 

Each team’s pivot acts like the pace car in a NASCAR race and controls the speed of the pack.  She also keeps an eye on the jammers, calls her team’s plays, and acts as a blocker.  The jammers are the only players that can score points.

The game is comprised of two thirty minute periods.  Each race is called a jam and lasts up to two minutes.  The referee blows a whistle and the pack takes off.  When the pack is 20 feet from the starting line, the referee blows the whistle again, and the jammers start to skate. 

The jammers do not score points during the first pass around the track.  They fight their way through the pack in an attempt to become “lead jammer.”  Lead jammer is established by the first jammer through the pack without committing a foul.

Once the jammers lap the track and re-enter the pack, they receive one point for each member of the opposing team that they pass legally.  While a jam lasts a maximum of two minutes, the “lead jammer” has the right to call off the jam at her discretion.

If a jammer passes all members of the opposing team and also laps the opposing team’s jammer, they score 5 points.  This is also called a “grand slam.”

source: howstuffworks.com & Memphis Roller Derby