For local use only you "primary domain" is localhost. You place your files in folders so you can test or build more than one site at a time so your actual "web address" becomes htttp :localhost/foldername There are probably fancier way to configure it but that is the easy way. If you want to play I can reccommend the xampp for windows non-installer version as it makes no registery entrys and does not auto load. When you want to play you start Apache and mysql. The rest of the time it is not running.
localhost only works where you think it would.... on the same computer. you should however be able to access your webserver via its lan ip too (generally 192.168.X.X). But neither of those is considered a domain name I wouldn't think.
but yeah, as kohashi said, localhost is the computer on which the service is running (if its your pc running apache, then localhost is your pc, if you have apache running on a computer on your network then localhost [to the 'eyes' of apache] is the machine on which it is running, if its a server on the internet then localhost to the 'eyes' of apache is the machine on which its running) also the local loopback ip address for localhost is 127.0.0.1 (so you can ping yourself, and do nmaps on yourself (this is different from the external ip address of your machine (eg the ip address of your netwoprk card - a good explination of what localhost is here - wikipedia /wiki/Localhost