A domain name is the unique name of a computer on the Internet that distinguishes it from the other systems on the network. They are sometimes colloquially (and incorrectly) referred to by marketers as "web addresses".

Every website, email account, etc, on the Internet is hosted on at least one computer (server). Each server has a unique IP address which is nothing but a set of numbers, such as "207.142.131.235". To access a particular internet service, one can specify its IP address in an appropriate application, such as an FTP client; however because it is difficult to remember numbers, an IP address can be associated with a fully qualified host name (a domain name), such as "www.wikipedia.org". Domain names also provide a persistent address for some service when it is necessary to move to a different server, which would have a different IP address.

Each set of letters and numbers between the dots is called a label in parlance of the domain name service (DNS). There are some rules about the size and make up of labels. Each must start with a letter or number, and then may be made up of letters, numbers, and hyphens, to a maximum of 63 characters. These are the rules imposed by the way names are looked up ("resolved") by DNS. Some top level domains (see below) impose more rules, like a minimum length, on some labels. Fully qualified names are sometimes written with a final dot.

Translating numeric addresses to alphabetical ones, domain names allow Internet users to localize and visit websites. Additionally since more than one IP address can be assigned to a domain name, and more than one domain name assigned to an IP address, one server can have multiple roles, and one role can be spread among multiple servers.

The following examples illustrates the difference between a URL and a domain name:

URL: http://www.freelancefree.com/

Server name: www.freelancefree.com

Domain name: freelancefree.com

Subdomain: www

Second level domain: freelancefree

Top level domain: com

As a general rule, the IP address and the server name are interchangeable. For most internet services, the server will not have any way to know which was used. The big exception to this is for web addresses. The explosion of interest in the web means that there are far more websites than servers. To accommodate this the hypertext transfer protocol (HTTP) specifies that the client tells the server which name is being used. This way one server, with one IP address, can provide different sites for different domain names. Every domain name ends in a top-level domain (TLD) name, which is always either one of a small list of general names, or a ISO-3166 two character country code.

ICANN (Internet Corporation for Assigned Names and Numbers) has overall responsibility for managing the DNS. It controls the root domain, delegating control over each top-level domain to a domain name registry. For ccTLDs, the domain registry is typically controlled by the government of that country. ICANN has a consultation role in these domain registries but is in no position to regulate the terms and conditions and the operations of how a domain name is allocated or who allocates it in each of these country level domain registries. Since generic top-level domains (gTLDs) are governed directly under ICANN, all terms and conditions are defined by ICANN with the cooperation of the gTLD registries.

Domain names which are theoretically leased can be considered in the same way as real estate, due to a significant impact on online brand building, advertising, search engine optimization, etc.