A domain is practically just a name. You can buy a domain from a domain registar. Once bought the domain is unavailable for everyone else. You have to renew it after a certain amount of time. You can connect the domain to an IP address. The IP address is like the street address for a computer/server. This computer can store the files for a website.
You can also just keep the domain name and resell it. You can transfer the domain to someone by giving them the password for your domain registar. There are also services that makes sure the transfer is safe and that you get your money.
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