Reading recipes

So after brewing with kits for my first two batches, ive decided that its time for me to try brewing from the recipes that everyone posts on here (awesome selection). the only problem is i feel like i dont understand what half of the stuff the recipe is telling me (ex. IBU, Gravity's). Anyone now where i can read up on what all this means so i can start brewing from recipes rather than kits.

cheers all.

http://howtobrew.com/intro.html