Any true "Sex and the City" fanatic can probably tell me which SATC character was on the board of a co-op (and I challenge you to comment below). But can you tell me what exactly a co-op is? Probably not. Co-ops, or housing cooperatives, which originated in the late 1800s, are creatures of New York, although they do exist elsewhere. In essence, a co-op is a legal entity from which you actually buy a share of your building rather than your apartment explicitly. Essentially, you are buying a long-term lease and the right to sole possession of your apartment indefinitely.
Much like a social club, co-op owners, through their board of directors, choose who their neighbors are. The board must approve of all new tenants, transactions, and sales before a new person can move in, and it acts in other ways much like a landlord. Co-ops are generally cheaper than condos, have lower closing costs, and protect you from troublesome neighbors, but they're difficult to sell, harder to rent, and have high monthly maintenance fees.