The AmSTeX language is described in the book The Joy of TeX for nroff. AmSTeX is a TeX macro package, not a modification to the TeX source program, so all the capabilities described in tex(1) are present.
The AmSTeX macros encourage writers to think about the content of their documents, rather than the form. The ideal, not always realized, 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''.
AmSTeX is the official typesetting system of the American Mathematical Society, and nearly all of its publications are typeset using AmSTeX. The online version of AMS Math Reviews uses AmSTeX input syntax for display of mathematical material. AMS authors can provide editors with computer-readable AmSTeX files.
For authors more familiar with LaTeX, an AMS-supported package called amsmath.sty is is available.