RC as in radio controlled usually means that the operator has a wireless, hand-held controller that sends radio signals through the air to the vehicle to tell it to go, stop, turn, and do other maneuvers.
RC as in remote controlled is often meant to mean toy vehicles that have a wired, handheld controller. A tether line between the controller and vehicle transmits commands through the wires to the vehicle. Slot cars are also a type of remote control car with the wired controller connected to the slot car track instead of the vehicle.
However, a remote control RC can also mean wireless radio controlled or optical (light) controlled vehicles, also known as infrared.
An RC with optical control or opti-control is a type of wireless, infrared remote controlled vehicle. While radio controlled toys communicate through radio signals, infrared or IR communicates through beams of light. This is quite similiar to the remote control on a television or DVD player.
A radio controlled vehicle or a remote controlled vehicle (wired or wireless), especially a hobby-grade radio controlled vehicle is commonly referred to as an RC and might occasionally be called an RCV.