2 channel amps are typically A/B amplifiers, being analog in nature. They directly amplify the signal applied (from the RCA cables and head unit) and amplify it via the power supply and transistors. Benefits of A/B amplifiers are their sound quality, they have low distortion if they are designed well. They are about 50-60% efficient, the rest is lost to heat. They are larger than Class D amps, which poses problems in smaller cars.
Subwoofer Mono amps (typically Class D) are designed to provide power in a smaller size and with less heat than an A/B amp, which makes them a great choice for subwoofers because of the power needed for them and the current draw needed to get that power. They basically sample the signal coming, and replicate it to the voltage rails in a digital type (binary) waveform. They are also cheaper per watt than an A/B amp. They are about 80-90% efficient, meaning less current draw for a given amount of power. Drawbacks are degraded sound quality due to crossover distortion and because the amplifier converts the analog signal to digital. It is hard to tell distortion from lower frequencies, so the casual user won't notice much of a difference between a good Class D amp and an A/B.
yup amps for subs are usually mono because they cover frequencies below about 100Hz, and in audio, you pretty much lose all stereo separation below about 250Hz anyway. being mono for bass doesn't lose anything.