To the untrained eye, roller derby looks like a bunch of people on skates slamming into each other whenever and wherever they want. Fortunately, there are rules to govern what parts of the body ("blocker zones") a skater can use to make contact with opposing players, and what parts of the opposing player ("target zones") can be legally hit. Unfortunately, many newer skaters (and even some veteran ones) don't take the time to learn these contact zones, and end up spending more time watching the game from the penalty box than they do on the track actually playing.
Rules about legal/illegal blocking and target zones fall under Section 5.2 CONTACT ZONES and Section 5.3 BLOCKING ZONE DIAGRAMS. Rules about illegal contact penalties fall under Section 6 PENALTIES.
Blocking Zones in a Nutshell: You can block an opponent using any part of your body from your shoulder to mid-thigh, except for your elbows, forearms, and hands.
Target zones in a nutshell: You can block an opposing player anywhere between their shoulders and mid-thigh on their front or side, but you cannot block any part of an opposing player's back. EDIT: You actually can make contact with part of the target's back, in a slim area where the target's sides turn into the target's back (as shown in the image below). But be careful when making contact with this area; there's a pretty thin line between a legal block and an illegal back block. Thanks to Davie Darko for catching this!
Images from WFTDA: http://wftda.com/rules/20100526/section/5.