With tcolorbox:
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{kantlipsum}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage[breakable, theorems, skins]{tcolorbox}
\tcbset{enhanced}
\DeclareRobustCommand{\mybox}[2][gray!20]{%
\begin{tcolorbox}[ %% Adjust the following parameters at will.
breakable,
left=0pt,
right=0pt,
top=0pt,
bottom=0pt,
colback=#1,
colframe=#1,
width=\dimexpr\textwidth\relax,
enlarge left by=0mm,
boxsep=5pt,
arc=0pt,outer arc=0pt,
]
#2
\end{tcolorbox}
}
\begin{document}
\mybox[green!20]{\kant[1]}
\mybox{\kant[2]}
\mybox[red!20!white]{\kant}
\end{document}

tcolorbox integrates flawlessly with theorem packages and math. Further, it can produce more fancier boxes than one can imagine. I feel that this is the best option to adopt when one has to highlight text / math / theorems. Though I have defined a command, new environments can also be defined with tcolorbox. For details, texdoc tcolorbox or visit www.texdoc.net.
A sample preposition:
\documentclass{article}
\usepackage[most]{tcolorbox}
\tcbuselibrary{theorems}
\usepackage{kantlipsum}
\newtcbtheorem{Proposition}{Proposition}%
{breakable,colframe=red!50!white!50!black,fonttitle=\bfseries}{}
\begin{document}
\begin{Proposition}{This is my proposition}{}
\kant
\end{Proposition}
\end{document}
Earlier attempt:
Sorry for joining late. I would like to add to cmhughes's excellent answer. Though his reply is already accepted as the answer, I wish to share mine. My method is simple and direct where colors can be changed very easily.
\documentclass[11pt]{article}
\usepackage{xcolor}
\usepackage{lipsum}
\newcommand*{\mybox}[2]{\colorbox{#1!30}{\parbox{.98\linewidth}{#2}}}
\begin{document}
%=====================
\section{First section}
%=====================
\mybox{green}{\lipsum[1]}
\par
\noindent\mybox{red}{\lipsum[2]}
\end{document}

mdframedpackage. – Mar 05 '12 at 03:53mdframedyou may post a minimal working example (MWE) to show, what you’ve tried so far … – Tobi Mar 05 '12 at 04:06