When I had the same question this is where I found it:
http://www.talkandroid.com/7686-what-is-rooting-and-why-should-i-do-it-the-pros-cons-of-android-rooting/

Matt's comment looks like a good one too.

Depending on what device you have the rooting method will be different though.  Personally I suggest reading up on your device at [XDA][1]

Hope this helps


  [1]: http://forum.xda-developers.com