What is a domain name?
Okay, think of it this way, you own a recreational vehicle (otherwise known as a camper). That camper has a name that is unique, much like your domain name (camper=domainname). Now, you own the camper and you can put anything in it that you like. BUT, unless you have someplace to park that camper and "hitch it up" it's of no use to you or to anybody who wants to "visit". Are you following me here?
Now, you must go out and find someplace where you can park your camper because right now you're just driving around and NOBODY knows where you are. So you find a place to park, let's say a campground with complete hookups (campground=host). NOW, you're hooked up. BUT nobody knows where you are. So you need to tell all those folks who want to come see you, where you are. So you call up the camper location folks (they keep a complete directory of campers and where they're parked, in fact there are many directory folks but what's great is that they all share their information) and tell them, "Yo...you can find directions to my place HERE." And you tell them where they can find directions. Here's where it gets a bit tricky. (Directory folks=Domain Name Servers or DNS). The folks who own the campground give you precise directions to give to the directory folks (that way everybody uses the same directions because usually, there are other campers in the campground too). Typically, all you need to do is tell the people that you bought your camper (domain name) from where you're camping out (host provides DNS information) and they'll take care of the details.
If you don't trust the people who you rent camper space from, you can always go out and pay somebody else to keep track of where you're camping and they'll disseminate the information to the proper authorities.
Once everybody knows how to find your camper, you can put anything you want inside and if somebody's interested, they'll stop by and take a peek inside.
Bottom line...when you purchase hosting space, the host will give you some DNS information that will look like either a strange URL and/or some numbers (IP numbers xx.xxx.xx.xx). Then you MODIFY DNS INFORMATION with the registrar (godaddy, verisign, register.com, whoever....). Hopefully you registered your domain name with somebody who doesn't require that you jump up and down on one foot and whistle dixie at the same time in order to change your DNS information. Godaddy is as easy as logging in with a user/pass and making the appropriate changes. Network Solutions (or whoever owns them now) requires that you submit a request, send an email, email back with the appropriate encrypted message...something like "the false albacore walks alone" or "what's the frequency kenneth?"...then you must walk around your neighborhood backwards with nothing on but your skivvies while chanting some Benedictine monk prayer. Finally, by the next full moon (as long as it isn't on a Friday) your information will be updated. If your lucky! Well...I guess it's not that tough but it sure ain't easy.