/newcommand

\documentclass[letterpaper]{article}

\newcommand{\tcol}[1]{\textcolor{blue}{#1}}

\begin{document}

A \tcol{command} usually begins with a backslash.

\end{document}

==================
General Usage
==================

\newcommand{cmd}[args]{definition}
\newcommand{cmd}[args][default]{definition}
\renewcommand{cmd}[args]{definition}
\renewcommand{cmd}[args][default]{definition}

These commands define (or redefine) a command.

cmd

A command name beginning with a \. For \newcommand it must not be already defined and must not begin with \end; for \renewcommand it must already be defined.

args

An integer from 1 to 9 denoting the number of arguments of the command being defined. The default is for the command to have no arguments.

def

If this optional parameter is present, it means that the command's first argument is optional. The default value of the optional argument is def.

definition

The text to be substituted for every occurrence of cmd; a parameter of the form #n in cmd is replaced by the text of the nth argument when this substitution takes place.