Xvfb is an X server for X11, but one that uses virtual memory to create a virtual frame buffer so that it can run on machines with no hardware display devices. Screens with various resolutions and bit depths can be created for the user's particular purposes.
More information can be found at Ubuntu manpages, as the man page will only be available with man xvfb when it is installed. (It is not installed by default in Ubuntu.)
In the package description for xvfb, it is noted that:
The primary use of this server was intended to be server testing,
but other novel uses for it have been found, including testing clients
against unusual depths and screen configurations, doing batch processing with
Xvfb as a background rendering engine, load testing, as an aid to porting the
X server to a new platform, and providing an unobtrusive way to run
applications that don't really need an X server but insist on having one
anyway.
Example
Run Google Chrome in a virtual frame buffer
Xvfb :99 -screen 0 1024x768x24 > /dev/null &
export DISPLAY=:99 && google-chrome -start-maximized http://example.com > /dev/null &
