At best, standard-definition shows have 4-channel Dolby surround sound (left, center, right, and one rear "surround" channel), but they usually have 2-channel stereo. High-definition broadcasts, on the other hand, support 5.1-channel Dolby Digital surround sound (left, center, right, left and right surround, and a subwoofer--the .1 part, for booming bass). This is similar to the sound you hear in a movie theater or, with the properly equipped home theater, on a DVD. Not all HD broadcasts have 5.1 Dolby Digital yet, but the majority do, and you'll hear--and feel--the difference. Of course, you'll need a home-theater audio system to take full advantage of it.