As far as I can tell, stress and traction seem to refer to similar ideas. According to Chapter 8 of Twiss and Moore's Structural Geology, traction is

Force per unit area on a surface of a specified orientation (Twiss, 129)

while stress is

A pair of equal and opposite tractions acting across a surface of specified orientation (Twiss, 129)

I don't understand this latter concept. Why have a term at all if the forces are equal but opposite? Then they should cancel out and be irrelevant right?

What is stress and how is it different from traction?