Packages
A LaTeX package is a macro that adds functionality to LaTeX and these are programs
that usually have the .sty extension. There are about a half dozen packages that are
commonly used and they automatically load many other packages. The interdependencies
among packages is non-trivial. All packages are loaded in the preamble of the article
with the usepackage command .
amsmath
This package is already loaded if you use the AmS document class "amsart". If you use the
LaTeX documentclass article then you need to load this package. The amsmath package
offers you many mathematical constructs that are not part of plain TeX and first appeared
Michael Spivak's program AmS-TeX.
amssymb
Whether you use the AmS document class "amsart" or the LaTeX document class
"article" you need to load this package which gives you access to a great deal of
symbols and AmS fonts.
amscd
If you need to produce a commutative diagram (to chase around) using the
commutative diagram environment then you need to
load the amscd package.
makeidx
If you want to have LaTeX make an index for your document and you are using the
LaTeX documentclass "article then you need to load the
makeidx package .
amsthm
If you want to cross-reference your theorems, Propositions, Lemmas, Definitions, etc.;
have your theorems typeset in various styles; and use the convenient proof
environment then you need to load the amsthm package and include the
theoremstyle and
newtheorem commands. In the case of the AmS
documentclass "amsart" the package is already loaded but you still need the theoremstyle and
newtheorem commands.
graphics
If you want to include graphics objects into your document with the
figure environment and
scalebox command then you need to load the
graphics package.