Signal to Noise Ratio - S/N
Ratio of the level of the signal to that of the noise, usually in terms of peak values in the case of impulse noise and in terms of root means square (RMS) value in the case of random noise. (The measurement of how 'clean' the sound is.)

Total Harmonic Distortion - THD
A measurement of harmonic distortion given by the ratio of the power of all the harmonics observed at the output of the system which are produced because of it's nonlinearity, when a signal-frequency sinusoidal signal of the specified power is applied to the input of the transmission system concerned, to the power of the fundamental frequency measured at the output of the system. (The measurement of how accurately the sound is being reproduced.)

Frequency Range
Upper and lower effective, or operating, frequency limits of a piece of equipment. (How high and how low (in terms of treble and bass) of a sound can be accurately reproduced)

As you can see, the technical definitions can be quite confusing. Here are the simple versions in longer form:

Signal to Noise ratio (S/N), how clean the output of a device is: If you have your game and it has killer sounds (like Half-Life), the total amount of sound coming from your soundcard would be a combination of the actual audio that is supposed to be there and the noise that is being added by the electronic components of the system itself. Less noise, more great sounding games and music!

With S/N, the higher the number the better. A quick tidbit of info, which I think is neat. The absolute maximum signal to noise from a pure digital device is 96db. This is considering that the sound used is 48KHz, 16-bit, because, you get 6db of signal for every bit. (16 x 6 = 96) So stating that, any board that is advertising greater than 96db S/N (Signal to Noise) is more than likely either A) testing from Line out to Line in or B) Testing the straight path between the analog CD input to the Line out. Even knowing that, the higher the number, the better. Another note about S/N: only if you are testing or listening through a very precise amplifier and speaker system, would you be able to tell the difference that 4db of S/N makes.