"localhost" is IP address 127.0.0.1, but that's a non-unique address. Assuming that you're on a LAN with TCP/IP, you'll also have a unique TCP/IP address as well, which (depending on OS) can be determined using a command named something like "ipconfig", "ifconfig", or (windows 9x) winipcfg.
So if your machine is at IP 216.199.21.14, they could access a web server on your machine as "http://216.199.21.14".
If you're on a DHCP subnet, however, this IP address is assigned dynamically and subject to change (particularly when you reboot). On a Windows network, you may be able to avoid this by referencing the machine using its Windows network name (
http://RODNEY, if your machine name is "RODNEY".
This is exactly how the Internet works. If your installation has a domain name of "id.com", and you have been identified to a Domain Name Server, people could pull up pages at "http://rodney.id.com" - "www" is just a common alias for the webserver's machine name.
Of course, all this is assuming that no firewalls are blocking people.