Background image..One image file with mulitple images?

Okay I am trying learn some web design technique and for the life of me I can't figure out how they are doing this. This website (http://www.adventnet.com/) has this file

http://image.adventnet.com/adventnethomepageimages.gif


as it's background image. But the one .gif file has all the images needed for the site. Where can I find a tutorial to learn this technique. It's kinda cool. I'm thinking advanced CSS has something to do with it.

I'm thinking this has something to do with negative margins, which pull back the background image to display only the part needed

you specify a width and a height, then the background-position
http://www.w3schools.com/css/pr_background-position.asp
or do it all in 1 description:

background: url("images/bg_tabs2.gif") no-repeat 100% -35px;

the -35px moves the image 35 pixels up so you cant see the top 35px.

-Adam