PWRs keep water under pressure so that it heats, but does not boil. Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.

BWRs actually boil the water. In both types, water is converted to steam, and then recycled back into water by a part called the condenser, to be used again in the heat process.

Since radioactive materials can be dangerous, nuclear power plants have many safety systems to protect workers, the public, and the environment. These safety systems include shutting the reactor down quickly and stopping the fission process, systems to cool the reactor down and carry heat away from it, and barriers to contain the radioactivity and prevent it from escaping into the environment. Radioactive materials, if not used properly, can damage human cells or even cause cancer over long periods of time.

Images and text courtesy of http://www.nrc.gov