pledges and bids

Just a transition period. You learn all about your new sorority and stuff, and then you are initiated a few weeks/months later, and you learn all the secretive ritual stuff about your sorority, and then you are an official active member. During the new member period you are in that sorority, like you can say "I am a Tri Delt", or whatever your sorority is, but you aren't an active member until you are initiated.

No you are not automatically in the sorority if you receive a bid. You're not "in" until initiation. This is so that if you receive a bid to a sorority you do not see yourself fitting in to, you do not need to go through initiation and you can rush again next fall. However, if you accept the bid and go through initiation, you CANNOT go through rush again at another school or in another year. Some girls have accepted bids, lived the sorority life for a month or so, but decided it wasn't for them and decided to drop. They aren't considered members of that sorority.