I made one last video on this subject to kind of clear up a common wording mistake when it comes to slipping. HarryC actually reminded me about the proper terminology for using a SIDE slip for landing in a crosswind. I always get the the term sideslip mixed up with forward slip. They have similar control inputs but they are used for different reasons. A sideslip is exactly how HarryC and several others have described so I wont go into that. A forward slip on the other hand is a cross control maneuver used to drastically increase drag and therefore is useful for increasing descent rate without increasing airspeed dramatically. This very useful if you have a field where you need to make a steep descent over an obstacle or you lose an engine to high to make a normal descent. I found a good article describing the difference.

http://en.wikipedia.org/wiki/Slip_(aerodynamic) I am sure there are better sources but this was just a quick internet search.

And of course a video demonstrating a forward slip with an engine out.
[youtube]http://www.youtube.com/watch?v=AQQNjIDeq5M[/youtube]