Lamport's original LaTeX had a separate program (SliTeX) for producing slides; it dates from the age when colour effects were produced by printing separate slides in different-coloured inks, and overlaying them, and was just about acceptable back then. When LaTeX2e came along, the reason SliTeX had to be a separate program went away, and its functionality was supplied by the slides class. While this makes life a little easier for system administrators, it does nothing for the inferior functionality of the class: no-one "who knows" uses slides nowadays.
The 'classic' alternatives have been seminar and foils (originally known as FoilTeX). Both were originally designed to produce output on acetate foils, though subsequent work has provided environments in which they can be used with screen projectors (see below).
The advent of MicroSoft PowerPoint (feeble though early versions of it were) has created a demand for "dynamic" slides - images that develop their content in a more elaborate fashion than by merely replacing one foil with the next in the way that was the norm when slides, foils and seminar were designed.
The prosper class builds on seminar to provide dynamic effects and the like; it retains the ability to provide PDF for a projected presentation, or to print foils for a foil-based presentation. The add-on package ppr-prv adds "preview" facilities (that which is commonly called "hand-out printing"). The HA-prosper package, which you load with prosper, mends a few bugs, and adds several facilities and slide design styles.
Beamer is a relatively easy-to-learn, yet powerful, class that (as its name implies) was designed for use with projection displays. It needs the pgf package (for graphics support), which in turn requires xcolor; while this adds to the tedium of installing beamer "from scratch", both are good additions to a modern LaTeX installation. Beamer has reasonable facilities for producing printed copies of slides.
Ppower4 (commonly known as pp4) is a Java-based support program that will postprocess PDF, to 'animate' the file at places you've marked with commands from one of the pp4 packages. The commands don't work on PDF that has come from dvips output; they work with PDF generated by PDFLaTeX, VTeX LaTeX, or dvipdfm running on LaTeX output.
Pdfscreen and texpower are add-on pakages that permit dynamic effects in documents formatted in "more modest" classes; pdfscreen will even allow you to plug "presentation effects" into an article-class document.
A more detailed examination of the alternatives (including examples of code using many of them) may be found at Michael Wiedmann's fine http://www.miwie.org/presentations/presentations.html
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=slidecls