Lauren,
ARGH! I just wrote out this long response and IE ate it!
I'll give you the short version
Formal recruitment (the proper term - but I will always call it rush
) is what is most talked about in the rush forum. You have several rounds of parties. During round 1 you will visit every sorority, and at each, you'll meet a few sisters, and they may sing a song or do a skit. Then some groups will invite you back for round 2, etc. until the final round (preference) where you will go back to the 1 or 2 (or possibly 3) groups you "fit with" the best. Then, if all goes well, you'll be matched with one of those sororities.
Informal rush is generally a lot more laid back. Not all sororities hold informal, so by waiting for informal you automatically eliminate some of your options. Generally, a sorority will hold 2-3 get-together type events - study breaks, picnics, etc. You go to as many as you like and stay for as long as you like. Then they will let you know if they are extending you a bid or not. (Note: At my school, most sororities holding informal rush only have 2 or 3 bids to extend, so participation in informal rush is by invitation.)
I advise you to go through formal rush. It's not a good environment for a shy person (I know firsthand
) but if you go in prepared with questions to ask, you'll be fine. If you receive a bid, great. If not, you will have a better idea of what sorority life is like, and you will have met lots of people with whom you can keep in touch - which will help when it's time for informal rush.
Good luck, and be sure to keep us posted