An intruder is someone who enters a place or situation despite not being invited. If a Girl Scout comes to your door to sell you cookies but ends up trying to watch TV with you, you can call her an intruder.
Some intruders are robbers who break into homes or businesses with the intent to steal things: "The intruder was charged with trespassing, and later was found to have stolen some jewelry and cash." Other intruders might instead invade someone's privacy. Intruder comes from the verb intrude, from the Latin root intrudere, "to thrust in," a combination of in- and trudere, "to thrust or to push."