You know what they say about concentrated evil? Yeah, that's radiation but more so.
Radiation isn't that bad, some types of fire alarms use decomposing isotopes (often alpha partiles of Americium) to know when stuff is smoking but... Well, crash course in decomposing atoms (i.e. radiation) 101!
Radiation is basically flung off sub-atomic particles from unstable nuclei. Most atoms that exist in nature usually are fairly stable ('fairly' being quite subjective) thus you don't have much radiation from them. The stuff you have to worry about is the synthetic atoms we have created, by throwing together other atoms to produce even larger atoms of entirely new substances. Most substances greater then atomic number 84 (Polonium) are usually the man made types - however not all are, and some substances larger (some smaller) also give off most kinds of radiation yet that's a fairly accurate number regardless.
There's three 'main' types of radiation - there's other types but we don't need to worry about them for this excercise.
There's Alpha, Beta and Gamma Rays (in order of size and how much it hurts your body).
Alpha is the biggest and is the size of Helium atoms; so it's fairly large thus only occurs with large atoms. It, like I said, is used in fire alarms. In a nutshell, it's so large that smoke blocks the alpha particles from reaching one side of the fre alarm, thus setting it off.
Beta is smaller, the size of a electron (and a positron, but that's another story) and has the medium penetrating power and the medium ionising power. Sort of the guy between the two - you really don't like him though. He's bad news bears even if he isn't gamma.
Finally, Gamma Rays are the worst. Electromagnetic radiation of high energy produced when matter is converted back to energy though some 'interesting' sub-atomic reactions. This stuff goes through everything, except a blackhole I guess, and usually wrecks some very intricate DNA ladders in your body. Thus, cancer.
Hope I helped a bit! If you want to know much else, I'll be here all week.
