DynDNS offers a variety of services, including a free service to map a fixed URL to the dynamic IP address assigned to your DSL/cable modem. The fixed URL takes the form of "yourname.mumble.xyz". You get to pick "yourname" (assuming yourname is not already taken) and choose from a long list of serious-to-humorous domains names like "dnsalias.com", "dyndns.biz" and "is-a-geek.net".
That handles getting translating your URL into a dynamic IP address. The next step is keeping DynDNS informed of IP address changes (which can happen without notice). While you can do this manually, there are attractive automated alternatives. Your DSL/cable router may offer this feature; my Linksys WRT54G v5 router does this. Or you can run
DNSUpdate on your homebase Mac.
Finally, if your homebase Mac is "behind" a router (often bundled with the DSL/cable modem), you must set up port forwarding in the router to direct Internet requests thru the router to your Mac. Setting up port forwarding is a topic for another thread.