The SliTeX command typesets a file of text using the TeX program and the SliTeX Macro package for TeX. To be more specific, it processes an input file containing the text of a document with interspersed commands that describe how the text should be formatted. It produces two files as output, a Device Independent (DVI) file that contains commands that can be translated into commands for a variety of output devices, and a `transcript' or `log file' that contains summary information and diagnostic messages for any errors discovered in the input file.
SliTeX is a version of LaTeX designed for making color slides, though you can use it for black-and-white slides as well. You don't need a special printer to make color slides; SliTeX uses the same black-and-white printer as LaTeX. You get color slides by copying SliTeX's output onto colored transparencies.
For a description of what goes on inside TeX, you should consult The TeXbook by Donald E. Knuth, ISBN 0-201-13448-9, published jointly by the American Mathematical Society and Addison-Wesley Publishing Company.
For a description of SliTeX, you should consult Appendix A of "A Document Preparation System: LaTeX" by Leslie Lamport, ISBN 0-201-15790-X, published jointly by the American Mathematical Society and Addison-Wesley Publishing Company.