You are confusing a couple of things. One, your domain will be registered no matter where you register it with the same reliability. Whether or not the registrar is fast, allows for easy changes, and has acceptable terms is another thing. These are the primary issues when choosing a domain registrar. Uptime is not one of them.
Now, the companies you mentioned (namedemo and namezero)also provide the DNS for the domains they register (for you, in their name). The DNS is what points the domain to the place where your files are on a server somewhere.
Generally, your host provides the DNS service and takes care of this but there are services available (for free even) that let you take control of this.
Next we come to how the site is set up on the server. IN some cases, the server directs visitors to the proper place by NAME (IP-less name based hosting). Sometimes the domain is set up with a dedicated IP address and the DNS is pointed to that (not just the server IP address).
Registering a domain (for the most part) gets you one thing. A name that you can call your very own, for a year (or however long you register it for, and keep paying the fees). It doesn't give you an IP address (host issue), nor have anything to do with uptime (DNS provider and host issue - could be the same company, could be different).
A real domain is blahblah.com/.net/.org/.nu/.etc.
[Edited by Chicken on 12-09-2000 at 07:58 PM]