about: is an internal URI scheme (also
known as a "URL scheme" or,
erroneously, "protocol") in various
web browsers to display certain
built-in functions. It is not an
officially registered scheme, and has
no standard syntax.
In early versions of Netscape, any
address beginning about: which wasn't
recognised as a built-in command would
simply result in the text after the
colon being displayed. Similarly, in
early versions of Internet Explorer,
about: followed by a string of HTML
(e.g. about:hello world)
would render that string as though it
was the source of the page - thus
providing a similar (though more
limited) facility to the data: URI
scheme defined by RFC 2397. Still
other versions of Netscape would
return various phrases in response to
an unknown about: address, including
"Whatchew talkin' 'bout, Willis?" (a
catch phrase from the TV show
Diff'rent Strokes) or "Homey don't
play dat!" (from a recurring skit on
the TV show In Living Color).
The most commonly implemented about:
URIs are about:blank, which displays a
blank HTML document, and simply
about:, which may display information
about the browser. Opera uses URIs
beginning with opera: for similar
purposes, and many about: URIs will be
translated into an appropriate opera:
URI if entered; an exception is
about:blank, which is not translated.
Source