If the "subtitle" attributes are to be the same from those of the title, and you want the information to appear right after the title, you can use the optional argument of the sectional units:
\documentclass{article}
\begin{document}
\tableofcontents
\section[Layout -- This section describes the layout]{Layout}
\subsection{Main Window}
\subsection{Footer}
\section[Navigation -- Here you will get information on foobar]{Navigation}
\subsection{Basic structure}
\subsection{Overview page}
\end{document}

As tohecz mentions in a comment this will also add this subtitle to the header marks.
Another sometimes used format is to write this "subtitle" right below the title for the sectional unit. The following code shows one way to achieve this; the idea is to define a new command \sectionsubtitle which writes the information to the ToC, immitating the way section entries are typeset, but suppressing the sectional and page numbering, and the leading dots; additionally, with this approach the subtitles won't be added to the head marks:
\documentclass{article}
\usepackage{etoolbox}
\makeatletter
\let\@nodottedtocline\@dottedtocline
\patchcmd{\@nodottedtocline}{\hbox{.}}{\hbox{}}{}{}
\patchcmd{\@nodottedtocline}{\normalcolor #5}{\normalcolor}{}{}
\newcommand*\l@sectionsubtitle{\@nodottedtocline{1}{0em}{1.5em}}
\makeatother
\def\sectionsubtitle#1{%
\addcontentsline{toc}{sectionsubtitle}{\protect\numberline{}#1}%
}
\begin{document}
\tableofcontents
\section{Layout}
\sectionsubtitle{This section describes the layout}
\subsection{Main Window}
\subsection{Footer}
\section{Navigation}
\sectionsubtitle{Here you will get information on foobar}
\subsection{Basic structure}
\subsection{Overview page}
\end{document}
