The product of Literate Programming. A literate program combines code and documentation; the important things are (1) to think of the documentation as central, not as a tacked-on addendum, and (2) to write the code for the benefit of the human reader, not the compiler.
The best known Literate Program is probably Tex The Program, written by Donald Knuth. Knuth invented Literate Programming. Another major one is "lcc", a small ANSI C compiler.
Another good example is Doc Tests for the Python Language.
Much discussion is on the Literate Programming page. It should probably stay there.
See original on c2.com 