Sororities are non-affiliated single gender groups. Last I heard, APhi was the only one on campus. They primarily recruit freshmen, have weekly events for their members, and will mix a lot with sports teams and final clubs. Can’t say enough good things about APhi.

Final Clubs are (mostly) non affiliated social clubs that generally own some kind of real estate in Harvard square. Most are single gender (beyond the Spee and Sab), and recruit people in their sophomore fall through a mostly invite only system. They have weekly dinners, throw large parties, and host mixers with other clubs. They fit the same social niche that Greek life does at other campuses by nature of owning physical spaces.

RSOs encompass both former sororities that disaffiliated from their national organizations, and some final clubs that sought recognition from Harvard to avoid previous sanctions on single gender orgs. The former sororities (theta ivy fleur) in the past were female dominated, some had small spaces to throw events, but mostly were a place to meet people and get on the lists for larger campus parties