No difference aerodynamically. The problem comes when trying to relate the manoeuvre to some ground reference.
Forward slip: Crossed controls are used in such a way that the flight path remains unchanged from the previously balanced flight path.
Example: (Nil wind for convenience) A slip is flown to maintain final. Previously the a/c was maintaining its flight path parallel to its longitudinal axis. A/c is yawed 'away' & rolled 'towards' the previous flight direction. The a/c maintains its original flight path but now with a new & different heading.
Sideslip: Crossed controls are used in such a way that the flight path deviates by some angle from the previously balanced flight path.
Example: (Nil wind to maintian commonality with the forward slip example) A slip is flown to impart a lateral component to the flight path eg to correct a displacement from final. The a/c is rolled 'away' & yawed 'towards' the original flight direction. The a/c maintians its previous heading. This results in a new & different flight path but with its original heading.