Almost everything during the "rush" process you are allowed to know- other than how a particular chapter votes and ranks rushees.-
Formal rush is very structured- you have an alotted time to meet many sisters of all the sororities over a few day time frame- this is a time to ask questions, bring up concerns, get to know people and have a great time. Since you are just learning about rush it is important to note you do NOT have to accept a bid! It is a personal choice- Formal rush is really a mutual selection- both you and the chapters narrow down choices.
Formal rush is a lot of fun- As everyone else has said read the things in the "rush" forum and go to your greek advisor on campus- they would love to give you more information.
Informal rush is less structured and honestly I don't know much about it- I am a new member who went through formal rush this year- but I think it is more of an invite thing- you invite friends who are interested in joining and other people inquiring about your chapter- but I could be wrong-
If you want to know- ask a greek
Who knows maybe Greek Life is for you
So go check it out (sorry this is long winded)