The system for doing these translations is DNS, the Domain Name Service, which is run by computers called nameservers. DNS resolves names into numbers. It also reverse-resolves numbers into names, so your computer can tell you the name of a computer it's talking to. Summary The main points are Computers use numbers to refer to each other. People prefer to use names.
Domains are a way of naming computers.
The Web is only one of many uses for computers named this way. It is if you're thinking of getting a domain name for your website. However, domains are a way of naming computers, not websites. If your website is the only website on a computer, it doesn't matter whether you think of the name as applying to the computer or to the website. However, if your website is just one of the websites on a computer, then you had best understand how that affects the naming of your website before you get a domain name.