The domain your business uses is how the Internet refers to it. As an example, Function Refined LLC uses "functionrefined.com." A professional business should have a presence online.
Some domain names may already be registered, and another one will have to be used. Domains are not forever. Every few years, the domain rights expire and must be renewed. The current owner can generally pay for the rights to a domain well in advance. Before the domain expires, the owner has first rights to re-register the domain.