The LaTeX language is described in the book LaTeX - A Document Preparation System. LaTeX is a TeX macro package, not a modification to the TeX source program, so all the capabilities described in tex(1) are present.
The LaTeX 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 LaTeX is the LaTeX manual referenced below, and the local guide in the file local-guide.tex or local.tex or some such.
elatex is the e-TeX extended mode version of LaTeX format.
lambda is the Omega version of the LaTeX format.
pdflatex is the pdfTeX version of the LaTeX format.
On some systems latex209 and slitex are available for compatibility with older versions of LaTeX. These should not be used for new texts.