Modern Internet protocols began being developed in 1973 with the development of the first Transfer Control Program (TCP). Internet Protocol (IP) addresses were first required on ARPAnet in 1983. With the birth of the World Wide Web in the early 1990s, the number of servers exploded, and a system other than numerical access became necessary. Domain names map a text-based string to a server residing at a particular IP address. For example, entering "http://www.google.com" in to a web browser maps the information request to the IP address 220.127.116.11. Domain names proved to be popular because they were easy to remember.
Top Level Domains
Top-level domains are the portion of the domain name furthest to the right. Top level domains include .com, .net, and .biz. National domain names like .uk for the United Kingdom, .ca for Canada and .tw for Taiwan are also top-level domain names. Top-level domains are assigned and controlled by the Internet Assigned Numbers Authority (IANA).
Second Level Domains
Second-level domains are the portion of the domain that precedes the top-level domain. For example, the "yahoo" in yahoo. com or the "google" in google.com. Second level domains are controlled and officially assigned by Internet Corporation for Assigned Names and Numbers (ICANN).
Sub-domans are the portion of the domain to the right of the second-level domain---for example, "mail" in mail.yahoo.com or the "blog" in blog.example.com. Sub-domains are assigned by a second-level domain administrator.
Before a domain can become active, it has to be registered and associated with a particular IP address. Domain registrars register domain names with ICANN and point the domains to the particular server that will be accessed when a user types the domain name into the web browser.
Overall control of domain names resides with the international organizations IANA and ICANN, but through the use of domain registrars, the registration and pointing of domains has become an easy, decentralized process.