Class A/B amps are also multi-channel amps. These are generally used on non-subwoofer speakers (your regular internal speakers and tweeters) due to their low THD ratings. A THD rating is basically the distortion ratio, a low THD rating will produce a cleaner amplified signal. This is very important if you are trying to achieve optimum SQ (Sound Quality).
Class D amps are Mono, or 1-channel, amps designed specifically to run subs. They tend to have a higher THD rating then most Class A/B amps, but it is not as diminishing to the SQ of bass frequencies as it is mids/highs frequencies. Most Class D amps also only come with a Low Pass filter (only allows frequencies under the filter setting to be amplified/sent to the sub), where as most Class A/B amps have both Low and High Pass filters (high pass filters work the same way, except they only allow frequencies over the filter setting to be amplified/sent to the speakers).
Class A/B amps also tend to have higher minimum ohm stability levels then Class D amps. Most Class A/B amps are only 2ohm Stereo/4ohm Mono stable, where as most Class D Mono amps are at least 2ohm Mono stable.
Class A/B - you can use on all speakers, including subs
Class D - you would only want to use on subs
There are other class amps out there as well (but most are derrivitives of either Class A/B or Class D)
