AC-3 is the most common version containing up to 6 total channels of sound, with 5 channels of full-bandwidth of 20 Hz - 20 kHz-normal range for speakers (front left and right, center, front, right rear and left rear) and one dedicated output channel for low frequency sounds known as low frequency Effect, or subwoofer. The Dolby Digital format also supports the use of Mono and Stereo. The AC-3 is one of the so-called perceptual compression formats. What it does, basically, is to remove all parts of the original sound, encoded analog, which can not be perceived by the human ear. In this way, it is achieved that the same information is smaller and therefore take up much less physical space. Once achieved compression of the original wave, you can add more information that was not previously possible.
The AAC file type is primarily associated with 'MPEG-2 Advanced Audio Coding File'. Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme for digital audio. AAC is promoted as the successor to the MP3 format by MP3’s creator, Fraunhofer IIS. AAC’s most noted use is as the default audio format of Apple's iPhone, iPod, iTunes, and the format used for all iTunes Store audio (with extensions for proprietary Digital Rights Management (DRM) where used).