A domain name is a word or word sequences that are entered into the address bar of an internet navigator to visit a particular website. In simple terms, a domain name is an online identity of a web owner.
A domain name helps to translate IP addresses to words. A domain name is a name that identifies one or more IP addresses, for instance the domain name microsoft.com represents about a dozen IP addresses.
Internet is based on IP addresses, not domain names, so every Web server requires a Domain Name System (DNS) server to translate domain names into IP addresses. Domain Name System stores and associates many types of information with domain names and helps to translate domain names to IP addresses.
Internet users use domain names as the URLs, ‘Uniform Resource Locators’. Domain names are used in URLs to identify particular Web pages. Take the URL http://www.pcworld.com/index.html, in the URL pcworld.com is domain name.
Every domain name has a suffix, which is termed as top level domain (TLD). The top level domain indicates the category to which a domain name belongs to. There are a limited number of top level domains, such as .gov (government agencies), .edu (educational institutions), .org (organizations, .mil (military), .com (commercial business), .net (network organizations), .ca (Canada), .uk (United Kingdom), .de (Germany or Deutschland), .us (United States) etc.
Technically, domain names are host names. Oftentimes domain names are referred to as "web addresses" that is technically incorrect and often domain names and hostnames are used interchangeably, but there are subtle technical differences between the two.
On the Internet, a hostname is a domain name assigned to the host. It is usually a combination of the host's local name with its parent domain's name. For instance in URL “fr.wikipedia.org," the host name is “fr” and the domain name is ‘wikipedia.org’. The host names are translated to IP addresses via the local hosts file, or DNS. A single may possibly have several host names. A domain name is not a host name, if it doesn’t have an IP address. Thus all hostnames are domain names, but not all domain names are hostnames.