Display address?

["Erik P. Olsen" <erik@xxxxxx>]

I know the display address usually (always?) is :0.0 and I would like to 
understand what the elements of this address mean. Would someone be kind enough 
as to point me to where I can read about this subject?

Thanks.
--
Erik P. Olsen, Civilingeniør, MSc
Solsortvej 30, DK-2000 Frederiksberg, Denmark
Phone: +45 38346480, Fax: +45 38346470, Mobil: +45 40765300