What are twisted pair Cables ? Why are they Twisted ?

Twisted pair cables, as the name suggests, are cables where a pair of conductors are twisted.  The pair of wires are twisted in order to cancel out the effects of electromagnetic coupling. 

The twist rate or the pitch of the pair refers to the number of twists per metre.  When two twisted pairs are lying together over large distances, there is a possibility of coupling between the two.  Hence, the twist rates of two pairs laid together should be different.