Amps
From the line devices, the sound path goes to:

POWER AMPLIFIERS
All amps raise a small input signal is to a higher level.
   Preamps: raise source level to Line level.
   Power amps: raise the line level signal to speaker level.
      from -20db to 0 db range up to about 20 db or so.

Amplifier Inputs:
Most power amps accept input signals of -20 to 0 db.
   Since sources are usually located close by:
Power amps usually have unbalanced inputs.
   At line level, usually little interference from induced voltages.

Amplifier Outputs:
Usually set to match input impedance of the speaker load.
   Typically 8 ohms (the impedance of a typical speaker).
   May also be taps for 4 and 16 ohm outputs.
   Allows for various combinations of speaker networks.
We will discuss speaker networks later.
May also be a 70 volt output tap.
Primarily for very long speaker runs.

Power rating.
Amount of power in watts that can deliver to the speaker.

Two figures are generally given:
RMS power or Root Mean Squared: average power continuously generated 
into a given impedance with an unvarying input of 0 db.
Peak power level: highest power level that can momentarily be delivered.
   MUCH higher than the RMS level.

It is important to understand the difference.
Take two amps, one rated 100 watts RMS, and one 100 watts peak.
100 watts RMS: delivers 100 w. continuously.
100 watt peak: delivers only a fraction of that as RMS without damage.
If manufacturer only says 100 watt amp, you won't know what it can really do.
The RMS value is the much more useful info.

Load impendance:
Load the amp is feeding when delivering the rated wattage.
Take a 100 watt speaker measured into 8 ohm load.
The amp will deliver:
   *About 50 watts or less into a 16 ohm speaker.
   *Over 200 watts into a 4 ohm speaker.
   *If you hook up a 2 ohm or smaller load, you will drive
      500 or more watts, and will probably destroy the amp.
So you need to know power in Watts RMS at a particular impedance, 
    i.e. the impedance of the speakers you are using.

Monoral vs. Stereo vs. Bridging Amps
Mono Amps: one output channel only.
Stereo amplifiers: really two amps in the case, one for each channel.

However, stereo amps may give you yet another option, Bridging.

Bridging: using two amps coupled in series as though they were a single amp.
The result: a single channel amp with the power and impedance of the two combined.

Two bridged amps rated at 200 watts into 4 ohms give you a single 400 watt amp into 8 ohms.

This can only be done with amps designed for bridging.