From-
http://www.bcae1.com/
Signal To Noise Ratio:
The signal to noise ratio is the difference between the noise floor and the reference level. The reference level is determined by the person making the measurements. For amplifiers, the reference may be, full power, one volt, one watt into a given load or any number of other things. For you to compare two pieces of equipment which were tested by different methods, you must know precisely what reference was used.
Common Misconception:
Many people believe that a piece of electronic equipment (equalizer, amplifier, crossover...) with a slightly higher signal to noise (s/n) ratio is infinitely better than one with a slightly lower s/n ratio. Many people don't realize how much a 30 or 40 dB ratio is. A piece of electronic equipment with a S/N ratio of 80dB may be good enough for all but the best systems.
(S/N Ratio is #124 on the right hand colum)