Well, a single channel amp has a single, lone output channel. Most amps of this design (not all, but most) are Class D amps designed for subwoofer duty.
A multichannel amp, by way of comparison, will have more than one output channel. 2-channel and 4-channel units being the most common but there are 3, 5, and 6 channel units out there as well. Most of the multichannel units are designed for interior speaker duty unless you're talking about the amps meant to be an all-in-one solution for an entire system and these, typically, will have some channels using a Class A/B topology for interior speaker duty and then 1 or 2 channels with a Class D topology for subwoofer usage.