most people say legacy ****. ones in stereo and ones mono (ie one channel but you can hook more than one speaker to it they just play the same exact thing)
A mono amp is meant to run one channel (usually one or two subs). A normal (stereo) amp is designed to run two seperate channels in stereo. Usually a stereo amp can be bridged into mono to drive one sub or two subs. Then you start getting into ohm ratings with the different configurations.