LaTeX has a great many useful commands, but it doesn't have a command to do absolutely everything, so if additional commands are required, they can be supplied in files called packages. [Documentation of packages] If you want to use any commands or environments that are defined in a package, you first need to specify the name of the package with the command:
\usepackage[<options>]{<package name>}
where <package name> is the name of the package
without the .sty extension, and <options> is a comma
separated list of options to be passed to the package (just as you
can do with class files using the \documentclass command).
Note that the \usepackage command must always go in
the preamble.
Let's look at a few examples.
\today
Nicola L. C. Talbot.
School of Computing Sciences.
University of East Anglia.
Last Modified: 2008-01-15