[Mach 1]

Yeah, and a relay does the same thing. A solenoid is basically a heavy duty relay.

I just finished an electronics class and the instructor explained what the only difference between a solenoid and a relay was, but I didnt really understand what he was talking about.

I think the solenoid has the armature drawn into the central hollow coil area, which usually isnt the case with a relay. The principle of operation is the same.

Like your door lock actuator would be a good example, the lock mechanism is retracted into the coil part of the soleniod. I guess a relay would just activate some contacts, and not have the moving part retracted or extended from the actual core of the coil inside.

The relay/soleniod on the strut tower brace used for starting is a relay, but might be constructed in a way where it could be considered a solenoid as well. i never tore one down.