how to interpret amp specs

Specifically, the higher db number in signal/noise ratio is better....but you have to consider the amplifier manufacturer's reputation and the particular model's place in the line-up.  You can't place a lot of emphasis on specs like that.  Look at two examples of 4 channel amps found at Sounddomain:

http://www.cardomain.com/item/USAUSC4065

http://www.cardomain.com/item/ABNA4004T

The first, a last-year's model USacoustics amp, is a well-designed piece of equipment whose specifications as published can be believed.  This model boasts a 100 db S/N ratio and I believe it.

The second example, the Audiobahn, also boasts a 100 db S/N ratio.  But I don't believe it.

THD is total harmonic distortion.  Any quality-built amplifier will have a low enough THD that its value would not make an audible difference.  A class D mono amp will have a higher THD than a class A/B amp, but using it in the way it was designed you wouldn't hear a difference.

In an actual installation, where there is an abundance of contributing factors such as signal interference, less-than-optimal grounding paths, improper setup, less-than-perfect head unit, and of course, attached speakers......these amp specs mostly mean nothing at all.  Even in a perfect world, you most likely would not be able to tell a difference between the output of two quality amps if one had a 100 db S/N and the other had a 96 db S/N.  Generally, if the manufacturer's writeup emphasizes flash and the "max" wattage is emblazoned in big numbers across the amp ( see this one ), you don't need to be looking at the specs that are listed.  They will tell you nothing.  You should, instead, target some reputable amplifier builders and look over their lineups.  Then, choose according to your pocketbook and needs.

To read up on what's what with amplifiers, http://www.crutchfieldadvisor.com/ISEO-rgbtcspd/learningcenter/car/amplifiers_glossary.html

...and this http://www.sound.westhost.com/amp-sound.htm