Roller derby is a high contact sport played by two opposing teams skating counter-clockwise on a track. Each team fields five members on the track at one time; four blockers and one jammer.
The game is played in two 30 minute halves made up of two minute segments known as a jam. At the start of a jam, four blockers from each team line up between what are known as the jammer line and pivot line to form a pack, while the jammers (the ones with the stars on the sides of their helmets) line up behind the jammer line (see figure A below).
You will hear an official yell, "Next whistle starts the jam! FIVE SECONDS!" At the sound of the whistle, the jammers take off into the pack and try to fight their way through; the first jammer to make their way out of the pack is known as the lead jammer. The objective is for the jammers to lap as many skaters as they can, scoring a point against each opposing skater passed. The jam can end early if the lead jammer "calls the jam" (the lead jammer slams their hands on their hips repeatedly).
You can't play roller derby without some consequences!
When a ref calls a penalty against a skater, they must serve 30 seconds of hard time in the box; if there is room, of course! Section 4.4.1 of WFTDA's rules states, "If a third Blocker reports to the Penalty Box while two Blockers for their team are seated, the third Blocker will be placed in queue and instructed to return to play. If a Blocker is standing in the Penalty Box, another Blocker may sit in the open seat." Additionally, if there is already a jammer in the box and the other jammer is called to the box, the seated jammer is immediately released.
Here is a list of common penalties (for a full list and additional information visit WFTDA):
Low block (below the knees)
High block (above the head)
Back block (hitting someone in the middle of their back)
Direction of gameplay
Out of play
Skating out of bounds
Blocking out of bounds
Use of forearms, hands, head, elbows
Cutting the track (jammer goes out of bounds and re-enters in an advanced position)
The yellow areas in the image below indicate legal target zones: