You can use \iffalse … \fi to make (La)TeX not compile everything between it. However, this might not work properly if you have unmatched \ifxxx … \fi pairs inside them or do something else special with if-switches. It should be fine for normal user text.

There is also the comment package which gives you the comment environment which ignores everything in it verbatim. It allows you to define own environments and to switch them on and off. You would use it by placing \usepackage{comment} in the preamble. Below is an example:

\usepackage{comment}
\begin{document}
\section{Multi-line comments}}
\begin{comment}
This is a comment,
a multi-line comment,
indeed.
\end{comment}
\end{document}