Vincent Zoonekynd's LaTeX website has some really straight-forward examples of chapter and section styles. I took the following minimal example by slightly modifying entry 31 of the section styles.
\documentclass{article}
\usepackage{lipsum,xcolor}
\makeatletter
\def\section{\@ifstar\unnumberedsection\numberedsection}
\def\numberedsection{\@ifnextchar[%]
\numberedsectionwithtwoarguments\numberedsectionwithoneargument}
\def\unnumberedsection{\@ifnextchar[%]
\unnumberedsectionwithtwoarguments\unnumberedsectionwithoneargument}
\def\numberedsectionwithoneargument#1{\numberedsectionwithtwoarguments[#1]{#1}}
\def\unnumberedsectionwithoneargument#1{\unnumberedsectionwithtwoarguments[#1]{#1}}
\def\numberedsectionwithtwoarguments[#1]#2{%
\ifhmode\par\fi
\removelastskip
\vskip 3ex\goodbreak
\refstepcounter{section}%
\hbox to \hsize{%
\colorbox{black!20}{%
\hbox to 1cm{\hss\bfseries\Large\thesection.\ }%
\vtop{%
\advance \hsize by -1cm
\advance \hsize by -2\fboxrule
\advance \hsize by -2\fboxsep
\parindent=0pt
\leavevmode\raggedright\bfseries\Large
#2
}%
}}\nobreak
\vskip 2mm\nobreak
\addcontentsline{toc}{section}{%
\protect\numberline{\thesection}%
#1}%
\ignorespaces
}
\def\unnumberedsectionwithtwoarguments[#1]#2{%
\ifhmode\par\fi
\removelastskip
\vskip 3ex\goodbreak
% \refstepcounter{section}%
\hbox to \hsize{%
\colorbox{black!20}{%
% \hbox to 1cm{\hss\bfseries\Large\thesection.\ }%
\vtop{%
% \advance \hsize by -1cm
\advance \hsize by -2\fboxrule
\advance \hsize by -2\fboxsep
\parindent=0pt
\leavevmode\raggedright\bfseries\Large
#2
}%
}}\nobreak
\vskip 2mm\nobreak
\addcontentsline{toc}{section}{%
% \protect\numberline{\thesection}%
#1}%
\ignorespaces
}
\makeatother
\pagestyle{empty}
\begin{document}
\lipsum[1]
\section*{Introduction}
\lipsum[1]
\section{Suite}
\lipsum[1]
\section{Suite}
\lipsum[1]
\section{Fin}
\lipsum[1]
\end{document}
This is what it looks like:

I would imagine this is what you're after. Moreover, you can easily change this if you want a different colour for your sectional backgrounds.