Roller derby is a full-contact sport played on quad roller skates. Two teams with five players each skate counter clockwise around an elliptical flat track. Here's a video that summarizes the game in less than a minute.
Each game is called a bout and lasts for 60 minutes. A bout is made of two 30-minute periods that are further broken down into two minute jams. Each team has five players on the track, three blockers, a pivot and a jammer. The jammer is the player who scores points for their team. The blockers and pivots job is to to make it as hard as possible for the opposite teams jammer to score points while at the same time help their own jammer. Players can only hit the opposing team with their shoulders, hips, and torso. Players who block illegally, start fights, or otherwise break the rules face penalties which can include anything from time in the penalty box to not getting to play the rest of the bout.
The team with the most points after 60 minutes wins.
First of all there's the pack. The pack is made up of pivots and blockers from both teams. At the start of the jam, the pack lines up in front of the jammers. A referee or timekeeper will signal the start of the jam with a single whistle and the pack will start skating with the jammers trying to get through the pack.
Then there's the jammer. Recognized by the star on their helmet cover, the jammer is the player that scores points for the team. In order to score, the jammers must break through the pack and skate one lap until they reach the pack again before they can start scoring points.
Although the first lap determines no points, the jammer that breaks through the pack first becomes the “lead jammer.” As lead jammer they have the power to stop the jam before the maximum two minutes have passed. To call off a jam, the lead jammer repeatedly hits their hips with their palms up. After that initial lap the jammer scores a point for every blocker they pass from the opposite team. The jammer can get a maximum of four points for each lap.
Each team has three blockers on the track, standing with no helmet covers on. Blockers play important offensive and defensive roles, helping their jammer get through the pack while at the same time working together to block the opposing team’s jammer. Blockers not only have to keep track of the two jammers on the track, but also make sure that no one is straying too far from the largest group on the track, otherwise the refs will call "No pack!" and the blockers must immediately reassemble.
Standing with the blockers is a pivot. Recognizable by the line on their helmet cover pivots are blockers with a special feature; if their jammer is struggling the pivot can take the jammer's helmet cover and put it on to become the active jammer, and can continue to score points. This is known as a "Star Pass."