A usurper is someone who wrongfully takes someone else's place. A usurper usually tries to take someone's place on the throne, rather than someone's seat on a bus.
If you were married, and someone cheated with your spouse and then married your spouse, you would probably consider that person a usurper: someone who took your place in a way that's shady or wrong. A king who is overthrown would consider the new king a usurper. Usurper is a strong word for someone who has taken or usurped what rightfully belonged to someone else.