Your domain name registrar is not your Web host

It's understandable that people confuse their domain name registrar with their Web host. They are related, but they are not the same thing.

There are two people you need to talk to in order to have a live website: 1) a domain name registrar, and 2) a Web host. 

Domain name registrar

The company where you buy your domain name, where it is stored, and where you manage the Web host it points to. Your Domain name registrar stores a) the location of your Web host on the Internet, and b) possibly where your email is hosted. 

Web host

The company where your website resides and where all the necessary files are stored (pictures, text, HTML code, etc.). Your domain name is pointed here by your domain name registrar.

Analogy

Your first and last name is like your domain name
Where you actually live is like your web host

I can look up your name in the phone book, but if I want to get together with you, I have to go to your street address. 

Web 101

(if you want all the gory stuff)

The technical address used by the Internet to locate your website is really a long number; i.e. 172.216.256.23. Because numbers can be hard to remember, we use words, or domain names, to find sites; i.e. highgatecross.com. And every domain name has one of these matching numerical addresses.

There are special computers stationed around the Internet (domain name servers) that match your domain name with the numerical address when someone is looking for your site.

Besides selling you a name, your domain name registrar keeps a record of the matching Web host's address that the domain name points to. If the address changes (you move your website to another host), it's the job of the domain name registrar to update all the domain name servers with your new address.

A word of advice: be able to access the the login for your domain name registrar and the login for your Web host at all times.

When you first purchase your domain name your registrar stores these locations, or addresses, on their name servers. They are your "authoritative name server," or the primary keeper of this information. 

Name servers = card catalogs

Think of a name server like card catalog in a library. If you are looking for the title Moby Dick, the card catalog will just give you a number for the location of the book. Likewise, your registrar has a "card" with your domain name and the address of your website and email.

There are lots of names servers 

There are other name servers throughout the Internet—probably even one at your ISP (Internet Service Provider), like AT&T or Comcast. Chances are they will also have a "card" for your domain name and website address. The question is, how did they get it?

Name servers at work

Well, your authoritative name server, usually where your domain name is registered, is responsible for propagating your "card" to all the other name servers around the Internet. That way, when your browser sends out a request for the location of a website you are looking for, it doesn't have to go all the way back to your authoritative name server. It may just find it locally at your ISP, saving time. 

Note: It is possible, but advisable only if you know what you are doing, to assign the role of authoritative name server to someone other than your registrar, but they will then be responsible for the location of your website, etc., and propagating it to all the other name servers.

Related: How the Web works