Also often abbreviated as Tx, this is the thing that actually takes your commands and sends your receiver the commands for the servos and the motors.
There are many Transmitters available on the market nowadays, and the low-cost ones start around $30. However, remotes for RC airplane or helicopter pilots, with programmable features, model memory, and computer simulators can start around $150 and go to nearly $3000.
All RC Remotes have a certain frequency that they operate on. Older, analog remotes operate on lower frequencies, which were determined by a crystal. Modern, digital, remotes usually operate in the Gigahertz Range, often at 2.4GHZ.
RC airplane and helicopter remotes have control sticks, which are mostly self explanatory. Usually, one of the two control sticks on an airplane remote is the throttle control stick, and is not spring loaded in the vertical direction, which means it will not spring back when you let go. Remotes are usually available with this stick on either the left or the right side, and are sold as mode 1 or mode 2. RC car remotes have a trigger and a knob to control different things.
Most RC remotes have something next to the Control sticks called the trim. this basically slightly alters the position of your control sticks, and is useful for finely altering the position of the servos, or the starting point of a motor controller. Transmitters also often have reversing, which reverses the output signal of the control sticks. this is usually controlled by a series of switches at the bottom of the transmitter.
The number of channels a remote has shows how many things it can control. For example a three channel remote could control three things, such as three servos, three motors, or a combination of any of those. Motor controllers and servos receive the same type of signal.
Expo, Telemetry, Limited travel and Model memory are all things seen on better, more expensive transmitters, but are not necessary for the average project. Expo stands for exponential rates, so that linear movement of your control sticks will result in an exponential output signal curve. Telemetry transmits information from the vehicle (such as battery voltage, altitude, motor temperature, etc.) back to the transmitter and displays these on the transmitter. Limited travel means that by moving your control sticks to their maximum will only send out a 30% signal, or whatever you set it to. both of these features are beneficial to sensitive mechanisms and airplanes, so that one can maintain better control over them. Model memory simply allows you to store your trim settings in the transmitter, so that you can recall them for later use, or eliminate the need of having to readjust with every time if you have multiple devices. RC car remotes are often equipped with dual rates, expo, memory, and even telemetry.
Most Transmitters need to bind to your Receiver to ensure proper communications. How to do this depends on your Transmitter/receiver.
Many transmitters need 12v to operate, usually supplied by 8AA batteries. this is 8 batteries too many for me, so i got a small, 0.8Ah battery and hooked it up to my remote, so that i only have one battery to worry about, instead of 8 small ones.
A cheap, beginner-level Transmitter that i can recommend from personal experience is the HobbyKing 6S