Add the following to your preamble:
\usepackage{color}
\newcommand{\hilight}[1]{\colorbox{yellow}{#1}}
To highlight text in the body of your document, use
\hilight{this is some highlighted text}
Voila!
Of course, you can change the color from yellow to any of the supported colors. For a list of those, check:
http://en.wikibooks.org/wiki/LaTeX/Colors
UPDATE:
There is a problem with the previous method is that the color box does not wrap properly. Here's a much easier and robust way of doing this
\usepackage{soul}
To highlight text in the body of your document, use
\hl{this is some highlighted text}
Cite as:
Saad, T. "How to highlight text in LaTeX".
Weblog entry from
Please Make A Note.
http://pleasemakeanote.blogspot.com/2009/08/how-to-highlight-text-in-latex.html