The latest full ruleset is downloadable at the WFTDA website.
The objectives of roller derby are relatively simple. Each team fields a single point-scoring skater ("Jammer") whose object is to lap as many opposing skaters as they can. The remaining skaters who aren't scoring points work both on offense and defense at the same time -- to block the opposing Jammer and to clear a path for their own Jammer.
Each team fields five players at a time. Out of those five players, four are BLOCKERS and one is the JAMMER (point scorer).
The skater wearing the star on her helmet is the jammer. The skater wearing the stripe on her helmet is called the PIVOT. The pivot is commonly the pack leader and defensive play-caller, similar to football's middle linebacker position.
On the whistle, the pack and jammers may begin to engage each other.
WHY ARE THERE SO MANY REFEREES OUT THERE?
Much like in football, the ref squad internally divides areas of responsibility -- the front of the pack, the rear of the pack, scoring for one team, scoring for the other team, etc. The refs practice communicating with each other to determine the legality of action on the track.
DO SKATERS HAVE TO WEAR OLD-SCHOOL 'QUAD' SKATES?
Yes. Ever since its invention in the 1930s, roller derby has traditionally been a 'quad' skate game. Quad roller skates promote control and stability, and their smaller wheel base reduces the chances for skaters to get tripped up on each others' skates.
ARE THERE FIGHTS ON THE TRACK?
Very rarely. The WFTDA rules call for the expulsion of skaters participating in fights, which helps to keep the skaters and referees safe.
DIDN'T ROLLER DERBY USED TO BE PLAYED ON A BANKED TRACK, WITH RAILS? AT LEAST THAT'S WHAT I REMEMBER FROM TELEVISION...
Yes — for fifty years roller derby was played primarily on concave ("banked") tracks. These tracks were big and expensive and required reassembly as the derby skaters of the time barnstormed from town to town.
In 2001 and 2002, skaters in Austin, Texas, lacking (at the time) the budget for a banked track, created the first drafts of a modified ruleset to allow the same basic game to be played on a flat surface. The popularity of this style of play has been exploding around the world, as the 'play-anywhere' nature of the flat game has allowed skaters to learn the game without investing in a banked track infrastructure.
A handful of leagues in America play primarily on banked tracks (most notably in Los Angeles, Austin, and San Diego), while hundreds of leagues play the "flat track" game. Many skaters and a small number of leagues train for both styles of play.