Unlike a sidebar, a boxout does not need to be a vertical rectangle along one edge of the page.
Boxouts are very commonly used in magazines, but may also be seen in books and on web pages.
The function of a boxout is primarily to hold related information separate from the main essay while retaining some degree of connection.
On web pages they frequently contain hyperlinks to related articles.
In terms of design, boxouts help to break up the page and make it more visually interesting.