The internet uses strictly IP addresses for locating and delivering information. The words associated with websites are strictly for human use and for referencing common names in case the numbers chanage, as they are a physical address in some respects.

Think of DNS as the phonebook of the Internet. Just as you know a name of a business and not the phone number, you reference websites by name and not by number. Each and every time you need to locate a resource you type in a name and DNS returns the number to your computer. This is similar as to a business may change their phone number. If you looked in the phone book each and every time you called, and the phonebook was constantly updated, you would never need to remember or store the number. Also, the business would be able to change their number or have multiple numbers without any care by you.

DNS is provided usually by your service provider which gets updates from another server, on and on up the stream. When websites move or change addresses they simply need to change a master server and the changes are propogated down the chain to your ISP.