"about:" allows a link, when clicked on, to directly display information we would normally use features of the toolbar to display, such as cache or plugin information. The "about" command does NOT work with Internet Explorer browsers. Lets see the available information displayable using the about: command:
Putting it all together:
<a href="about:">Click to view browser information</a>
<a href="about:cache">Click to view cache information</a>
<a href="about:plugins">Click to view plugins information</a>
Now that you're armed with more link knowledge, go out and fortify your position on the WWW!