Radio control is the use of radio signals to remotely control another device. The term is used almost universally to refer to the control of model cars and airplanes from a user-held control box.
Radio control was origially developed during World War II, primarily by the Germans who used it in a number of missile projects. Their main effort was the development of radio controlled missiles and glide bombs for use against shipping, a target that is otherwise both difficult and dangerous to attack. However by the end of the war the Luftwaffe was having similar problems attacking allied bombers, and developed a number of radio controlled anti-aircraft missiles, none of which saw service.
The effectiveness of their systems was greatly reduced by British efforts to jam their radio signals. After initial overwhelming successes, the British launched a number of commando raids to collect the missile radio sets. Jammers were then installed on British ships, and the weapons basically "stopped working". The German development teams then turned to wire guidance once they realized what was going on, but these systems were not ready for deployment until the war had already moved to France.
Both the British and US also developed radio control systems for similar tasks, in order to avoid the huge anti-aircraft batteries set up around German targets. However none of these systems proved usable in practice, and the one major US effort, Project Aphrodite, proved to be far more dangerous to it's users than the target.
Radio control systems of this era were generally mechanical in nature. A small radio receiver was placed in the missile, with the signal from the controller received and "played" into a small speaker. In front of the speaker were a number of small metal "fingers" with different resonant frequencies, each one tuned to vibrate when a particular tone was played in the speaker. The vibration would push on elelctrical contacts connected to the control of the missile. The controller's radio would play the different frequencies in response to the movements of a control stick.
These systems were widely used until the 1960s, when the increasing use of solid state systems greatly simplified radio control. The mechanical resonant systems were replaced by similar electronic ones, and the continual miniturization of electronics allowed more signals, referred to as control channels, to be packed into the same package. While early control systems might have two or three channels, modern systems include 20 or more. Typical radio control systems for model aircraft employ pulse position modulation, and actuate the various control surfaces using servos.
Remote control military applications are typically not radio control in the direct sense, but take the form of instructions sent to a completely autonomic automatic pilot. Instead of a "turn left" signal that is applied until the aircraft is flying in the right direction, the system sends a single instruction that says "fly to this point".
Today radio control is used almost entirely by hobbyists.