If you apply rudder and opposite aileron then you are side slipping. Normally, the lower wing is on your windward side. You can do that to get pointed straight down the runway and counter drift in a cross wind. It is also a useful (and fun) way to drop some altitude - since it is a high drag configuration for the plane. You've got the fuselage at an angle to the relative wind, hence the high drag. I've never encountered the term forward slip other than to distinquish a straight ahead side slip from a slipping turn.

Here is an example of a plane you might have some trouble side slipping because you'd run out of clearance for the low wing tip as you approach ground: http://www.roedelmodell.de/deutsch/falke.html You'd have to crab it in, then straighten out with rudder as you flare and hold the wings level at the same time.