What are virtual packages?

[Jim Byrnes]

Utunbu 12.04 & libreoffice 3.5..

I am trying to run some example libreoffice python scripts and getting 
errors.  Google suggests that one possible reason is that 
libreoffice-script-provider-python is not installed.  It is not listed 
in Synaptic.  This page [1] says that it is a virtual package provided 
by python-uno.  Does this mean it is "built in"?

[1] 
http://packages.ubuntu.com/search?keywords=libreoffice-script-provider-python

Thanks,  Jim