Crosstalk is unwanted leakage of a signal from one channel to another. It can be caused by capacitative or magnetic coupling of the signal. More rarely it could be caused by leakage through poor insulation.
When a current flows through a wire, a magnetic field is produced around the wire. This magnetic field can induce a current in another nearby wire. Twisting wires together minimises the problem because equal and opposite currents flow very close to each other. This almost completely cancels out the induced magnetic field.
Computer network cables like CAT5e rely on the twists to minimise crosstalk. To minimise crosstalk over a wide range of frequencies, the twists vary along the length of the cable.
