Listen up fans and freshmeat, 'cause I'm only going to say this once.
There is no ball in derby.
Roller derby is a full contact sport played on an oval track. The bout (game) consists of two periods, each 30 minutes long. Each period is made up of jams (as many as will fit in a period). Jams are a maximum of two minutes long (but can be less) with a 30 second interval between jams. The lead jammer can call off a jam at any time.
The pack lines up on the track at the first start line with the two pivots in front. The jammers line up about 20 feet behind the pack on the jammer line. A single whistle blows to start the pack. A few seconds later a double whistle blow starts the jammers. The jammers sprint to catch up with the pack.
Now it starts to get interesting. The jammer comes up on the back of the pack and attempts to make it through the pack. The jammer is assisted by her own team members, while the opposing team blockers do everything legal (and sometimes not so legal) to keep the jammer from getting through.
The first jammer to make it through without incurring penalties takes on the status of lead jammer and can call off the jam at anytime. If no jammers make it through cleanly, there is no lead jammer, and the jam will last for two minutes.
Once the jammer is through the pack, she sprint around the track and catch up to the back of the pack—referred to as lapping the pack. This time (and every following lap for the jam) she will score one point for every opposing player's hip she passes. If she also laps the other jammer she gets a grand slam—5 points.
The jam ends with four whistles either when the two minutes are up or the lead jammer calls off the jam. If the jammer's lucky, she's survived to skate again. If not, well enough said.
Check out this video (Hammer City Roller Girls) to get an idea of how to play derby.