If your name is Fred you might name your computer fred. Nancy working at the next desk might name hers nancy. Then everyone would know whose computers fred and nancy are. We'll name Nancy Smith's computer nancy.smith and Nancy Jones' computer nancy.jones. We just invented domains.
Domain names are just family names for computers.
If computers really were named this way, smith and jones would be domains. In real life, of course, the family names are much more likely to be the names of the companies. But their function is still the same. They let one company name its computers without worrying about how other companies name their computers. Subdomains Real domain names on the Internet usually have more than one part, because large domains are subdivided into smaller ones, sometimes called subdomains.