In very simple terms, a servo is what moves something where it is installed. Usually in a remote place. I explain...

A signal is sent from the transmitter to the Receiver. The receiver decodes the signal into a "command" to the servo. The servo moves whatever is attached to it based on the command given by the receiver. A receiver is capable of controlling several servos. All this is powered by a local power source, usually a battery.

I hope this was simple enough.

Rafael