Following are sections on TeX documents, software and sites.
LaTeX is a set of macros built on top of the plain TeX package that simplify typesetting by letting the user concentrate on the structure of the text rather than on formatting commands.
The most complete sources of information for LaTeX2e are LaTeX: A Document Preparation System (2nd Ed.) by Leslie Lamport and The LaTeX Companion by Goossens, Mittelbach and Samarin. Be sure to obtain the second edition of Lamport's book since the first doesn't contain information about the LaTeX2e extensions.
Following are sections on SGML documents, software, and sites.