Before printing your "hot from the oven" 3D model, you'll need to convert it in a language comprehensible to your 3D printer. A 3D printer is a CNC machine with a extruder as printing tool, so the logical step was adopting the same programming language used in the CNC, which is GCode.
Basically it tells to the machine which dots should connect... yes, all machines work like games you have played as a kid :)
Well, after this brief explanation, let's take a look at Slic3r, it's just one of many STL-to-GCode translators developed for 3D printing. At first glance it seams more user friendly than Skeinforge, but lacking some options/features, but I guess the main reason is the development team behind Skeinforge is the same as ReplicatorG, the Makerbot creators!
So, why should I use Slic3r? Well, take a look at RichRap wrote an impressive, let's call an essay, about Slic3r, take a look at it!