The only difference between a forward slip and a side slip is what you are try to accomplish by slipping. In terms of aerodynamics, they are identical. A slip angle is a slip angle as far as the airplane is concerned.
The term "forward slip" is most often used to describe an intentional slip for the purpose of adding drag to steepen a descent without a resultant airspeed increase. The aircraft heading is offset from the intended ground track by the approximate slip angle. Pilots of some airplanes with limited forward visibility (such as a Pitts Special) also use this technique during final approach for improved runway visibility. You could think of this kind of slip as being "forward" along your intended ground track.
Side slips are generally intended to slip the airplane "side"ways with respect to the intended ground track for the purpose of countering a crosswind component during landing.
So to summarize, a slip is a slip and the aerodynamic effect is identical. Apparently someone at some time found it necessary to categorize them according to their intended purpose, probably for instructive reasons. You are not the only one ever to be confused by this awkward terminology.
I hope that clarifies more than further confuses your understanding!
westhawk