LaTeX macros can be used as simple text replacments for example:
\documentclass{article}
\newcommand{\LUG}{LEGO User Group}
\begin{document}
\section{Welcome to the \LUG}
\end{document}
More Advanced usage allows LaTeX macros to be used like functions:
\documentclass{article}
\newcommand{\LUG}[1]{\textbf{#1} LEGO User Group}
\begin{document}
\section{Welcome to the \LUG{tartan}}
\end{document}