it's called being two faced. When behind someone's back you show your real disgust or dislike for someone but pretend to be friends with them for any number of reasons. It's not "people" you are talking about here (referring to your statement that you don't get "people") it's fake and two faced people you don't get and there really isn't a reason to get it. They are people not to be trusted, so I'd just keep your conversation with such people at a minimum.