Sideslips and forward slips are very similar, but the previous explanation is correct.
The generally accepted definition is that the side slip aligns the longitudinal axis of the aircraft with the runway and utilizes bank angle to maintain track on the centerline (crosswind situation). The actual path through the air is to the side to compensate for the wind. Zero wind and you're not heading for the runway, but going to the side of it.
A forward slip involves applying rudder in one direction and enough opposite aileron to prevent a turn. In zero wind you will still be tracking the centerline of the runway. Usually you apply full rudder and enough aileron to compensate.
These are the standard definitions used by FAA, etc.
I think the main difference is that in the sideslip you are using enough aileron to control the drift and just enough rudder to keep the longitudinal axis aligned with the runway. In a forward slip you are using a whole lot (to the stop in a light airplane) of rudder and then aileron as required to prevent the turn. Consequently, you generally use a whole lot more rudder in the forward slip. I would agree with those that say that if you need to do this to lose altitude you've botched the approach and should just go around.