Both are aerodynamically the same thing, which is a slip. The difference is in what they are used for and the order in which you make the control inputs...

In a forward slip, your puropse is to hang as much fuselage in the breeze as possible to create drag and bleed energy. You start by apply full rudder and then add enough opposite aileron to keep your desired track.

With a sideslip, your purpose is to fly the airplane on (and aligned with) centerline in a crosswind. You first apply enough aileron to offset the crosswind, then you apply the required rudder to keep the fuselage aligned with centerline.