They're a family of snakes. Just like your Boas, Elapids, and so on. They make up the majority of snake species.
Also, to answer your original question, you can't go wrong with a Corn Snake. They've got almost everything you could ask for in a pet snake: good availability, simple care requirements, docile, and of course some morphs (if that's your thing). Mine has never been trouble; never bitten me, never even thrown a defense posture. They're great.