Slides use a bigger base font size, suitable for presentation material, and the slides class provides an easy way to make overlays -- a slide which can be laid on top of a previous slide to fill in certain gaps.
For an example see http://www.colorado.edu/ITS/docs/latex/Unix/examples/sampleC.html
The LaTeX project announced in LaTeX News 11 on June 1999, that the class will be unsupported in the future.