The RC receiver located inside the RC plane is part of the RC radio equipment. It will catch the signals from the RC transmitter, send those signals out to the servos, and speed controller that controls the aircrafts control surfaces and rpm. RC receivers are commonly operating on either the MHz frequency band or the GHz frequency band.
MHz RC Receiver
A RC receiver operation in the MHz frequency band (usually 27, 35 or 72 MHz) uses crystals to work on the correct frequency channel. The receiver needs a crystal equivalent to the crystal used in the transmitter. Attached inside the receiver and extending out of it is a wire antenna. Usually the antenna is about 1 meter and should be extended in its full length out of the aircraft in order to get the best range. Never try to shorten the antenna in any way since this will reduce the range in an uncontrolled way.
Since the introduction of synthesized receivers that binds to your transmitter’s frequency channel, the need for a crystal in the receiver does not exist.
There are some drawbacks with the MHz RC receivers, or more generally RC radio equipment working on the MHz frequency band since there can be interference with another set of radio equipment. This is why you need to make sure that no one else at your flying field uses the same frequency channel, i.e. crystals, as you.
The synthesized RC receivers do not solve this issue so there you also need to ensure that you exclusively are using your frequency band.
GHz RC Receiver
Usually the RC receiver operating in the GHz frequency band uses 2.4 GHz. The need for crystals in the receiver and transmitter is removed and some kind of spread spectrum modulation is used instead. This reduces or eliminates the interference issue seen in the MHz frequency band.
The antenna used on a GHz RC receiver is much shorter than the one used in a MHz RC receiver. Some receivers use a satellite receiver to avoid some radio problems. This receiver is a small receiver that is connected to s specific port on the main receiver and then placed in such a way that the antennas of the two receivers are orthogonal.