Adding a callout box (like a sidebar within a post) is a cumbersome
process if you try to do it by hand. You could code a stylesheet to
handle this, but this won't work for people who use feed readers to
read your articles.
Hence the need for this plugin. Define the styles you want to use
in the settings page, and then create a new callout box like this:
[callout title="A demo callout."]Callout content here.[/callout]
You can see it live at
http://componentoriented.com/wordpress/insert-callout