AMSLaTeX
What is AMSLaTeX?
The name "AMSLaTeX" is used to mean "LaTeX with AMS extensions". The AMSLaTeX extensions are part of the standard LaTeX distribution, and are also mirrored on CTAN.
This AMSLaTeXonly distribution is divided into two parts:

The amscls part contains AMS "document class" files and a theorem package that can be used independently. These provide the setup necessary to give a LaTeX document the general structure and appearance of an AMS article or book.

The amsmath part is an extension package for LaTeX that provides various features to facilitate writing math formulas and to improve the typographical quality of their output.
If you already have a TeX distribution from 2005 or later, AMSLaTeX will be included, and you do not need to obtain it separately.
Prerequisites

A working TeX system
TeX is not an AMS product. See the AMS TeX Resources page for sources.

A current working LaTeX system, dated June 1995 or later
LaTeX is not an AMS product. See the LaTeX Project home page for information if you do not already have LaTeX.
Download AMSLaTeX version 2
This distribution is in two parts:
These archives will unpack into an existing TEXMF structure, in most cases providing a readytouse installation.
Documentation for AMSLaTeX version 2

README file for AMSLaTeX 2.0: brief overview of the entire AMSLaTeX collection.

Installation instructions for AMSLaTeX 2.0 (copy of install.txt contained in the zip file)

amsmath (math features):

amscls (document classes  amsart, amsbook, amsproc  and theorem package):
Additional Documentation
Except for the Short Math Guide, the files listed below are included in the AMSLaTeX distribution, and should be on your system, unless documentation files were excluded from the installation.
This material is best understood when the source and output are examined side by side.
Auxiliary Packages

amsmidx.sty  Package for use with document classes based on amsbook.cls to produce multiple indexes. Instructions are in the file amsmidx.txt.

amsbooka.sty  Package for use with document classes based on amsbook.cls that contain chapters written by individual authors; this package will properly format tables of contents as well as the chapter headings. Instructions are in the AMS Author FAQ.
Old Versions of AMSLaTeX
Version 1.2
This version is provided, unsupported, for use only when exact output fidelity is needed for a preexisting document that was written using version 1.2 of AMSLaTeX.
Most AMSLaTeX documents written with version 1.2 will produce satisfactory output when typeset with version 2.0. The most noticeable effect of upgrading will probably be corrections in the vertical spacing for some equations.