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Domain Names
Suppose your friend calls you on the phone and tells you that they found this great electronics website. They tell you to type in "BestBuy.com" (not an endorsement, BTW) into your web browser. What they just described to you is a domain name. Each domain name needs to be registered before it can be used. There are a few registration companies that will handle this for you (for a fee of course).
To use an analogy suppose you were thinking about building a home. You need to find and purchase a land plot. When you find one and make the purchase, you need to register that purchase with your county or provincial government body. The domain registrar company is like the county government in this analogy. You could in effect, think of a domain name being analogous to land. You find an available domain name and register it with a domain name registration company. There are not that many registration companies and the two major ones are Namecheap and GoDaddy.