Define parallel forces and what's the difference between parallel forces and couple?

I can't think of a way to hint without giving you the full answer, so I'll aim to use enough jargon to make you think about what I'm saying.

Parallel forces are just what you expect, they are forces which are parallel, i.e. operating in the same direction. They may or may not be of the same magnitude. These forces cause acceleration in the direction of the forces, and will cause rotation if the forces are not equal.

A couple is a pair of forces of equal magnitude acting in antiparallel directions, i.e. they cannot produce a net force (or acceleration), but CAN produce a net moment, causing rotation.