In order for someone to connect to your VNC Server across the
Internet, they'll need a way to find you. The Internet has two
ways of doing this: first, most familiar, it uses "domain names", 
like "www.google.com" or "aol.com". When you type in an address
like that, the Internet relies on something called a Domain Name
Server (DNS) to translate that name into an actual IP Address
(see below). You too can get your own domain name, for free, with 
many "Dynamic DNS Providers". http://www.no-ip.com seems very 
popular with VNC users: just install their client on your VNC 
Server PC (Windows or Linux) and you're good to go.

The second more "direct" way of connecting with someone is to use
a numerical IP address. The trick is finding it out: with most dial
up or PPPoE DSL connections, your ISP address changes every time
you connect. To find out your current IP address "automatically",
just open a web-browser to http://www.whatismyipaddress.com. Then 
just tell that number to someone trying to connect with you, and 
you'll soon be connected.