Phi Sigma Sigma,
Alpha Omicron Pi is absolutely FANTASTIC
. Seriously, it sounds as if there are two or more sororities on your campus and their formal recruitment begins with Open House where Potential New Members can visit each house and get a feel for which ones they like and which they don't. After Open House (and I am guessing since I don't know where you are in school) the Potential New Members decide which houses they want to visit for the second party and the Sororities decide who they want to invite back. All that gets matched up and goes on for however many parties there are until Bid Day. You should keep an open mind and seriously consider each house. You may change your mind about where you want to go.
Do you mind telling us where you go to school?