A domain name is a unique label that defines an area of administrative autonomy, authority, or control in the Internet, based on the Domain Name System (DNS).

Domain names are used in various networking contexts and application-specific naming and addressing purposes. They are organized in subordinate levels (subdomains) of the DNS root domain, which has no name.

The first-level set of domain names are the top-level domains (TLDs). This includes the generic top-level domains (gTLDs) such as the well-known com, net and org, and the country code top-level domains (ccTLDs) like ph,eu, and jp.

Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users who wish to connect local area networks to the Internet, run web sites, or create other publicly accessible Internet resources. The registration of these domain names is usually administered by domain name registrars who sell their services to the public.

Individual Internet host computers use domain names as host identifiers, or hostnames.

Hostnames are the leaf labels in the domain name system usually without further subordinate domain name space. They appear as a component in Uniform Resource Locators (URLs) for Internet resources such as web sites.

Domain names are also used as simple identification labels to indicate ownership or control of a resource. Such examples are the realm identifiers used in the Session Initiation Protocol (SIP), the DomainKeys used to verify DNS domains in e-mail systems, and in many other Uniform Resource Identifiers (URIs).

An important purpose of domain names is to provide easily recognizable and memorizable names to numerically addressed Internet resources. This abstraction allows any resource to be moved to a different physical location in the address topology of the network, globally or locally in an intranet which usually requires changing the IP address and the corresponding translation of this IP address to and from its domain name.

Domain names are often referred to simply as domains and domain name registrants are frequently referred to as domain owners although domain name registration with a registrar does not confer any legal ownership of the domain name, only an exclusive right of use.

Reference: wikipedia.org