Modulation.

What is modulation?
Modulation is where one signal is varied in some way by another signal. The frequency of the signal being modulated should be several times higher than the signal being used to vary it.
The type of variation, determines the type of modulation. For example, Amplitude Modulation (AM)varies the amplitude of the main signal (known as the carrier).

Why modulate?
Low frequency signals, such as speech and music will not travel far, at least not without making everyone between you and your contact deaf.
On the other hand, radio waves can travel great distance and can be filtered (so that you only pick up the one you want). Unfortunately the radio wave, on it's own, is pretty useless.
So, what is needed is some way to hook the sound waves onto the radio waves, so the radio waves will carry the sound further. This is where modulation comes in - the radio signal acts as a carrier for whatever you really want to get across - rather like the box that your Corn Flakes come in. Just like the box, it is thrown away.

What are the different types of modulation?
There are many different types of modulation, but some of the main ones that you may hear about are listed below. For more info about one of the methods, just follow the link.

Amplitude Modulation: The main carrier signal (27MHz for CB) has it's amplitude varied by the signal from your microphone.

Frequency Modulation: The frequency of the main carrier is varied by the audio signal from your microphone.

Sideband: The carrier is usually filtered out, and the signal transmitted will just be either carrier + audio, carrier - audio, or both. The carrier is added again at the other end, by the receiver.

SSB: Single Side Band. Only one of the sidebands is transmitted - either upper or lower.

USB: Upper Side Band - SSB, where the signal transmitted is carrier + audio frequency.

LSB: Lower Sideband - SSB, where the transmitted signal is carrier - audio frequency.

DSB: Double Side Band - refers to signals containing both upper and lower sidebands. Where the carrier has been removed, it may be referred to as "suppressed carrier" DSB.