I'll be the first to bite....I'm probably going to get some of this wrong though...

a social GLO (any of the 26 NPC sororities, the IFC fraternities, and dozens of independent national and local groups) are greek letter organizations (GLOs) that were founded to form a sister/brotherhood among their members through social interaction and community service. Members can be of any major, any GPA (within limits, usually something like 2.5 or so), etc. They have a recruitment process in which the Potential New Members (PNMs) seek them out, and membership takes place by mutual selection. You can only be a member of one of these groups, however, you can also be a member of an honorary or professional org as well.

Professional GLOs are a varied bunch. They are generally focused on the advancement of a certain profession, and being of that major may or may not be a requirement. They may or may not be co-ed. They may or may not be as socially active on campus as the Social GLOs. Generally their recruitment is held independent of the Social GLOs, but that can vary. Same thing with dual membership...generally, you can be a member of both a professional and a social sorority, but that can vary too.

Honorary GLOs are somewhat similar to Professional GLOs. Generally, they are focused on one field (ie. History, Biology, etc). Members are usually sought out and invited on an individual basis for achievement in that field, rather than recruitment parties being held. Membership is non-exclusive (meaning that you can be in an honorary GLO and any other group as well.)

Hope that helps, and anybody please correct me if my info is wrong!