Domain name is the unique name that you choose for your web site. Web sites in the World Wide Web have their names and are identified by the domain names that correspond with numeric IP address.
By the following example we will illustrate the difference between URL and domain name:
URL: http://http://www.something.com/index.html
Domain name: http://www.something.com
Registered domain name: something.com

A complete domain name consisting of a host, the second-level domain, and the top-level domain is called FQDN (fully qualified domain name). For example, http://www.something is a FQDN, where www is the host; something is the second-level domain; and .com is the top-level domain.
A top-level domain (TLD) is the last part of an Internet domain name; that is, the letters that follow the final dot of any domain name. For example, in the domain name http://www.website.com, the top-level domain is .com (or COM, as domain names are not case-sensitive).

There are several types of domain names:
gTLD (generic top level domains) such as .com, .net, .org, .name and others
ccTLD (country code top-level domains) also called as regional domain names, for example: .fr, .it, .de, .eu, .nl, .be, .ru and others.
The company that is registering the domain names is called registrar, the person who registers domain name is called registrant.

When you register domain name it is recommended that you pay attention on the actual registrant of your domain name as some companies register domains on their names.

Domain name is always registered for a definite period; it is called domain name registration period
For most of domain names the minimal registration period is one year
Though for some domains (for example .uk, .mobi, .us) the minimal registration period is 2 years.
The date when domain name was registered is called registration date
Depending on the registration period, the domain name will have expiration date
Example: domain name was registered on 1st of April 2006
It was registered for 1 year
Thus domain name expiration date will be 1st of April 2007
It is always recommended that customers keep in mind their domain name expiration date and renew domain before domain expires in order to exclude any sort of downtime.

The information about the registrant of the domain name and its registration period are normally displayed in Whois database. Using Whois database you can determine whether domain name is available for a new registration or is already registered/taken, can determine the registrant (owner) of the domain name, domain name registration and expiration dates, IP address assigned to the particular domain name.