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• Surround sound is a term used to describe a type of audio output in which the sound appears to "surround the listener" by 360

Surround Sound is the term commonly used to describe a system in which the sound output appears to surround the listener — that is, the technology gives the impression that sounds are coming from all possible directions. Surround sound is a way to provide a more realistic and engaging experience. Sitting in front of your standard television or playing a computer game with basic $10 stereo speakers plugged in will provide you with sound. You hear the music, voices and other sounds coming to you from straight ahead. Using a surround sound system, however, you'll hear variances in the sound and and it will be presented to you from different directions. For example, the car driving off screen sounds like it is coming from the left. While playing a 3D shooter game, the rocket gun blasts sound as if they are being shot from overhead or grenades sound like they are exploding at ground level.
Today consumers demand immersive entertainment and surround sound plays a big part in delivering engaging media. To this end, there's no lack of options when it comes to surround sound technology and systems.

All technical aspects aside, Surround sound works because multiple audio channels are received through speakers that are positioned at various locations in the room. This is programmed into the source and the sound tracks are decoded when the source is played. While this may seem simplistic, it's important to remember sound systems started out with monophonic sound, a single channel system. In monophonic (mono) sound systems, the signal sent to the sound system is encoded as one single stream of sound — and the sound is usually received through a single speaker.

Advancements led to stereophonic sound (stereo) where the sound was split between two channels, left and right. Stereo sound enabled listeners to hear some ambiance of the production — for example, a recording of a live concert in stereo where you begin to hear playback from extraneous noises at the performance. Surround sound takes it a step further by producing a live-quality effect. If you're at an actual concert, you hear sound from behind you, and listening to a recording in surround can produce the effect of this noise — coming from behind you, or to the right, or even from above you and moving down. Surround sound adds realism and a new field of depth to your listening experience. This is accomplished not only because you have more speakers for output, but because the sound recording itself contains more audio channels.

The A/V (audio/video) Receiver
At the heart of every home theater is an audio/video receiver (A/V). Using a DVD movie as an example, the audio is encoded when the DVD is produced by packing multiple audio channels into a compressed format for storage. When you play the DVD movie, your DVD player or A/V receiver (commonly called a a home theatre receiver) decodes the encoding scheme (i.e. Dolby Pro Logic II for example). Decoding capabilities of an A/V receiver are built in. Most A/V receivers today can decode Dolby Digital and Digital Theater Sound (DTS), while higher-end receivers may also include DTS-ES or THX Surround. Today's A/V receivers allow you to control the video source and other video options, but for sound you'll need it to route the sound from different sources (TV, DVD, VHS), control bass, treble, and volume. You can amplify signals to the speakers, and modern A/V receivers also handle the all-important surround sound decoding.

Reference:
http://www.webopedia.com

• A standard for high-quality digital audio that is used for the sound portion of video stored in digital format.

• An individual discrete audio track. When referring to more than two channels it is called multichannel.

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