Crosstalk

Crosstalk refers to noise caused by signal coupling from one conductor to another nearby one. There are two physical mechanisms which create this coupling: capacitive and inductive.

In the former, the electric field allows the two conductors to act as the two surfaces in a capacitor; just as a normal capacitor allows high-frequency signals to pass through it, so too here.

In the latter, changes in the magnetic field around the first conductor, as current flows through it, induce current flow in the second conductor, somewhat similar to a transformer.