The total RMS power is the total output of the amp by multiplying each channel of power at its lowest operating impedence.
The peak power of an amplifier is how much power the amp can deliver at the highest burst of loudness in a song. This is diferent from the RMS or continuous rating of the amplifier. Peak is usually approximately 3 times the RMS although it really depend
Class D is a digital amplifier with a more efficient power supply. Class "D" is generally used for subwoofer applications. Class AB is an active resistance linear mode amplifier, used mostly in multi channel amplifier setups.
Crossovers allow you to adjust the frequency of your output.
A preamp output allows you to send a preamp signal to additional amps you may be running. In some cases, it can also allow you to use the crossover on your first amp to control the other.