With the news full of stories about radiation leaking out of the damaged nuclear plant in Fukushima, I was curious and researched radiation.
Here is a link to a Reddit thread which superbly explains what radiation is and how it kills the body. It also discusses cancer, what it is, and why it does damage. The explanations are in scientific terms, but also illustrate the concepts as if you were five years old. This is very worthwhile to read.