The X language is described in the book X - A Document Preparation System. X is a macro package, not a modification to the source program, so all the capabilities described in tex(1) are present.
The X macros encourage writers to think about the content of their documents, rather than the form. The ideal, very difficult to realize, is to have no formatting commands (like ``switch to italic'' or ``skip 2 picas'') in the document at all; instead, everything is done by specific markup instructions: ``emphasize'', ``start a section''.
The primary source of documentation for X is the X manual referenced below, and the local guide in the file local-guide.tex or local.tex or some such.
elatex is the e-*TX extended mode version of X format.
lambda is the Omega version of the X format.
pdflatex is the pdf version of the X format.
On some systems latex209 and slitex are available for compatibility with older versions of X. These should not be used for new texts.