A solenoid has a core that moves when electrical current is applied. The core physically comes in contact with the switch it actuates. A relay is any device that can remotely activate a device based on an electrical signal. All solenoids are relays but all relays are not solenoids. You can call the thing on the starter and by the strut tower a relay, but you can only call the thing on the starter a solenoid. Does that make sense?
