Noise

At some point in the mix some level of noise will be present, if your using analogue gear the effects will be more pronounced...
If You are using software and rendering to a compressed format such as mp3 noise will be produced that was not present in the higher quality original...

Noise is basically unwanted random frequency occuring from distortion of amplification of background sound already present. But noise can also be used to hide/mask other problems if deliberatly introduced in to the mix in the right way.
Another interesting thing about noise is that for some reson it makes us 'believe' what we are hearing is more real - as in that a song completely free of noise does not sound as pleasing as an iidenticle recording with a little noise introduced.

Noise is also used during sound rendering stage or file conversion ..the process known is as Dithering.