The domain is the address of your website. That is, the URL that users type into the browser to reach your page.
For example, the domain of this site is Your Site.
A site can have only one domain, and a domain can only be registered by one person (or company). The registration of a domain has the main purpose to "make sure" the exclusive use of that ' URL. However, it is in no way linked to whether you decide to create a site or not.