Why is noise is considered to be a "Common Mode Signal"?
Firstly, this picture below hopefully explains why a differential signalling receiver will cancel out interference (or noise) on both wires: -



It's a simple case of A minus B i.e. the noise/interference gets cancelled but the wanted signal gets left intact.

Secondly, when an interference source is some distance away from the two-wire cable, it largely inflicts noise equally onto both conductors hence the noise is called "common-mode". When a noise source is much closer to one wire than the other there will be a noticibly differential noise signal and this isn't so easily dealt with by a receiving differential amplifier.

So, you have common-mode noise and differential noise and to make the incident noise only (mainly) have common-mode effects you need to do several of the things below: -


Use matched impedances to ground so that any influence from noise sees equal impedances to ground thus, one wire does not naturally receive a larger noise signal than the other.
Keep the noise source as far from the cable as possible
Use differentail signalling to improve signal level amplitude (reduce SNR)
Use twisted pair so that magnetic noise induces the same voltage on both wires. This also helps a bit with electric field interference.
Use a screen so that electric fields couple to the screen and, due to internal capacitances, couple equally to both wires.
Use a receiver that can deal linearly with signal and superimposed noise (transformers are good for this).