Stereo sound is the most common way to listen to audio tracks. It consists of two channels of sound. In this case, the channels are separated into two separate cables. Combined signals, such as those carrying an RF signal, carry multiple channels of stereo sound. The tradeoff, however, is reduced quality of all of those signals.
Dolby surround is still the most common form of surround sound.
It consists of four channels that must be decoded from a stereo source. It also includes a mono rear channel that is presented through two rear channels. The center and rear channels are matrix encoded in a stereo signal. This means that a Pro Logic signal can be listened to as either stereo or surround sound. The rear and center channels are heard through the left and right channels of the stereo sound.