Also often abbreviated as Tx, this is what really has their commands and sends your receiver the commands of the servos and motors.
There are many transmitters (transmitter) available on the market today, and the cheapest start around $30. However, controls away from RC plane or helicopter, with programmable functions, memory model, and simulation software can start around $150 and go to almost $3,000.
All remote RC have some frequency operating on. Older, analog controls operate at frequencies lower, which were determined by a crystal. Modern, digital, remote controls usually operate in the GHz, often to 2.4 GHz range.