Most ISPs use DHCP, yes. Some ISPs (like mine), however use static IPs, which means they don't change.

I'm a little fuzzy on some of this, so don't take it as God's word (but you can think of me as God, if you wish ). One way you can be found on a dynamic ISP is through your DNS (domain name server) name. Even though your IP changes every time you log on to your ISP, your DNS name remains constant.

Things get a lot more complicated than that, but I'm no uberhacker, and I'm too lazy to go any further into it right now.