In general sororities are groups of women who share common goals and hang out/live together (if they have a house, but some don't). We have Social activities with each other (sisterhoods) and fraternities (mixers/socials/exchanges) We also do community service, sometimes in the form of fundraising, sometimes by donating time to a cause. It is a great way to make a large university environment feel a little more like "home".
In general there are three types of sororities NPC (national panhellenic conference) which is 26 sororities that have come together and have agreements which help each other out. these orgs. have to abide by these agreements and they all have chapters all throughout the US (and some in Canada) there are also organizations that have chapter all over the US but aren't part of NPC... (gamma sigma sigma, comes to mind) There are also Local organizations which only have chapters at that particular school. There are also NPHC (another council that has come together there are 9 groups in this council) organizations belonging to this council are historically african-American (although they are not exclisively for people who are african american) There are also other councils for Multicultural orgs.
Since I am a member of an NPC group I can really only tell you about that... you join through a process called "rush" or recruitment which lasts a week and you get to meet all the sororities at "parties" (no, not those kind of parties, just sisters and prospective new members or "PNM"s, people who want to join) at these partes you talk to the sisters and they will tell you about their organization and chapter and you can get to know them and they get to know you. I suggest you read the rush forum for more information on exactly how that works and I know there are threads about NPHC recruitment or intake that will give you more info about that. This thread may help as well
different recruitment styles GOOD LUCK and feel free to ask any questions you want because someone around here almost always has an answer for anyquestion you can think of !