If you buy a co-op, you would actually become a shareholder in a company that owns the building, but that shareholder ownership also gives you a permanent lease of the unit.

Co-ops require board approval for transactions, so a co-op board could just deny your purchase without giving any reason. And that opens a lot of doors for potential discrimination. The application package you submit to the board can be invasive, like resume, copies of your tax returns, letters of recommendations and such (some describe as more invasive than going to a dentist). They may even require an interview... for what's practically the right to rent a unit after paying a huge deposit.

Despite the difficulties, that may still be a financially sound strategy compared to the alternatives. If you don't plan on living there for many years, it's probably not worth it.