Yeah, use google. I mean, why come to a photography forum when you could just search the web? In fact, maybe they should just shut down the forum and we'll all meet over at google. I mean, nothing posted on this site can't be found somewhere else anyway.
Come on guys, if you don't want to answer newbie questions, then don't come in the beginner's section and click on threads titled "rule of thirds".
To answer your question (although allen already did, I guess), the rule of thirds is pretty simple. Divide your scene up into thirds both horizontally and vertically. In most cases, you don't want your focal point to be in either of the center thirds (the horizontal one nor the vertical one). Of course rules are made to be broken, so don't think this rule is concrete. There are cases where centered works.