Now, if you were to stand right outside somebody's house, right there in the eavesdrop, so that you could secretly listen to people inside, you'd be an eavesdropper.
Whether or not you're actually standing right underneath someone's eaves, if you eavesdrop, you listen in on somebody else's private conversation, and you're sneaky about it, so that they don't know you're listening.
Eavesdropping on people, or eavesdropping on their conversations, is a rude thing to do! No one likes an eavesdropper. If you're having a private talk with someone, and you find out later that someone else was eavesdropping on you, you'd probably be angry!