An explanation of Formal Recruitment can be found at:
http://panhellenic.tripod.com/FAQ.html
Informal recruitment (also known as Continuous Open Bidding) takes place all year long. It is less structured than Formal Recruitment and the interested women only visit those houses they are most interested in. However, only chapters that are below campus total can participate in COB. If you are going to a large campus with a large Greek population, chances are that most houses will reach campus total or go above it during formal recruitment. Until their membership drops below total, they will not be able to take additional new members.
There have been several threads about Rush, bid-matching, informal vs. formal rush, etc. on these boards. I've responded to many of the threads, so if you do a search for my user name (PnguinTrax) in the Greek Life and Rush forums you should come up with all those topics (and more).
Good luck!
Barbara