Literate programming is the combination of documentation and source together in a fashion suited for reading by human beings. In general, literate programs combine source and documentation in a single file. Literate programming tools then parse the file to produce either readable documentation or compilable source. The WEB style of literate programming was created by D. E. Knuth during the development of TeX.
The "documented LaTeX" style of programming is regarded by some as a form of literate programming, though it only contains a subset of the constructs Knuth used.
Discussion of literate programming is conducted in the newsgroup comp.programming.literate, whose FAQ is stored on CTAN. Another good source of information is http://www.literateprogramming.com/
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=lit