Exclusive housing development, generally. Usually the way a HOA states in the states is a developer will buy a huge plot of land, divy it up, and build houses on all of the plots. These are sold with a contract that stipulates you're in the HOA and you have to sell to people who will also join the HOA. If you break contract they can generally foreclose on you. They've become popular as people are overly worried about neighbors doing something stupid with their house that'll destroy property value, which personally I've never seen happen (the worst I've dealt with is my apartment complex mandates solid white curtains are facing the outside in all windows and doors). Many times they'll pick good people to run them and nothing major happens. Also in return for paying dues to this HOA, you'll get access to a community pool, a community park, possibly landscaping, possibly snow plowing, etc. Then sometimes what happens is nobody wants to participate and the craziest of the crazy start leading it, and you get all sorts of good juicy internet gossip. These houses also generally fill out the American McMansion stereotype as the develop wants to maximize profits so the yards are tiny, the houses are overly huge and the quality is non-existant.
Sometimes what can also happen is neighbors group together and decide to form a HOA. Those are even better for juicy internet gossip as its a mix of who joins and who doesn't, and the HOA sort of looks like a half completed jigsaw puzzle and they then start trying to enforce HOA restrictios on people who never joined.