Did you know that Facebook, YouTube, Google, or Apple don't know your passwords? They don't store them in their databases so that hackers can't retrieve them in case they manage to get in. You're the only person who knows them. But how can they match passwords when you log in, then?
Great tip: If the web service you use would email you your password, meaning that they actually store it, you had better run away from them. [Sploid]