Essentially, Tor is an world-wide network of relay servers integrated and designed to ensure the privacy of the surfers (not serfs) by anonymizing all browsing activities and locations. Reading the overview on this page will give you a basic understanding of how this works.
There are several ways that one can participate in the Tor network itself and you can read about those here.
For those of us not running servers or otherwise currently capable of participating on the network side of Tor, there is Vidalia. Vidalia is a client—available for Mac, Windows, and Unix systems of all flavors—which allows you to take advantage of the Tor network (as a proxy network) and thus annonymize much of your on-line activities.
For Opera users, I found this article for using Opera over Tor.
Tor is located in the Ubuntu repositories. (If you are not running Ubuntu as your operating system, I recommend it.) This is a good place to start if you are interested in either using or participating in the Tor network.
I’m just doing my little part to make the world a better place. I hope that I can help you work toward that same goal.