A domain name is the “friendly” name of a website. If you want to get technical: A domain name defines a realm of administrative or authoritative control over industry and location-specific subject matter.
Domain names are for human beings to reference a desired website. Imaging trying to remember Toms River Web Design is located at http://188.8.131.52 !
Domain names are also an opportunity to come up with a clever (as in branding) or strategic (as in SEO) wording to represent a business. Domain names may be aliased so that multiple names refer to the same website.