A top-level domain is a domain name’s “last name”. For Norwegian domain names, it is .no, for Swedish ones .se, etc. In addition to the country code top-level domains, there are also so-called generic top-level domains, such as .com, .org and .google. In principle, a top-level domain can be .anything.
The international organisation IANA is responsible for delegating top-level domains. On their webside, you can see a complete list of all the top-level domains in the world. Norid used to publish a copy of this list, but that is no longer the case.
Go to IANA's list of all the world's top-level domains