The following classes are distributed with LATEX:
article class described in LaTeX: A Document Preparation System.
book class described in LaTeX: A Document Preparation System.
report class described in LaTeX: A Document Preparation System.
letter class described in LaTeX: A Document Preparation System.
slides class described in LaTeX: A Document Preparation System,
   formerly SLiTeX.
article.
   Formerly the proc package.
article.
article.  The document you are reading
   now uses the ltxguide class. The layout for this class is likely
   to change in future releases of LATEX.
article. The layout for this class
   is likely to change in future releases of LATEX.
\normalsize.  It is principally intended for debugging
   and testing LATEX code in situations where you do not need to
   load a `full' class such as article. If, however, you are
   designing a completely new class that is aimed for documents with
   structure radically different from the structure supplied by the
   article class, then it may make sense to use this as a base and add
   to it code implementing the required structure, rather than
   starting from article and modifying the code there.