You will get just about everything you need when you buy your radio system. Most aircraft and helicopter systems (of four or more channels) come with transmitter, receiver, three or more servos, rechargeable NiCad batteries for both the transmitter and receiver, a charger for both transmitter and receiver batteries, switch harness, frequency flag and extra servo control arms.
Sometimes referred to as the 'TX'. The transmitter is the hand held control box that converts your control movements into electrical signals and sends them via radio waves to the receiver in your model.
Sometimes referred to as the 'RX'. The receiver is the small electronic box the size of a matchbox and placed in your model. The receiver converts the signal from your transmitter into electrical control signals which can be sent to your servos.
Servos are the devices in the model which actually produce the control movements. Servos convert the electrical signals from your receiver into physical movement to control your model. A different servo is needed for each control function or radio channel.
Virtually all 4 channel systems come complete with NiCad rechargeable battery packs (a pack for the transmitter and a pack for the airbourne receiver and servos) and charger that will recharge both packs. You should NEVER use Alkaline batteries in any R/C model aircraft.
When looking for a radio, you will find that people often speak about different types of modulation. They are referring to the way the electronic control information is sent from your transmitter to the receiver over radio waves.
Amplitude Modulation, was the first means of modulation in R/C. The control information is transmitted by varying the amplitude of the signal. This form of modulation has, in R/C aircraft been superseded by FM.
Frequency Modulation, is now the usual method of transmitting the radio signal. It is less prone to interference than AM. It is transmitted by varying the frequency of the signal.
Pulse Code Modulation, FM is still used, but the control information is in the form of a digital signal rather than the pulse width used in AM or FM. Using PCM adds additional safety and is very much less prone to interference.