"Slip'er" wrote

I heard references to "forward slip" to get rid of altitude on
approach.

Both prior posts pretty much nailed it. A forward slip can be made
either to the left or right. Basically, you intentionally use cross
controls (rudder & aeleron). I believe that a forward slip generally
refers to using a slip to keep the aircraft aligned with the runway
for landing in a crosswind. This could mean landing on one wheel to
maintain directional orientation. This is my preferred approach when
flying in a cross wind in a taildragger.

A side slip is similar but in this scenario, the slip is can be more
aggressive. A side slip is used to shed altitude.

I do believe that you have them reversed..........

From http://www.airbum.com/articles/ArticleSlips.html

The old fashioned forward slip is one of those maneuvers that on one hand
would appear to be redundant to modern flap systems. At the same time,
it's one of those basic maneuvers that if understood and practiced gives
the pilot yet another tool enabling him to put the airplane exactly where
he wants it on approach.

Unless we're talking about the so-called side slip in which a slipping
motion to the side is canceled out by the crosswind so the airplane
tracks straight.

Bob Moore
ATP CFI