»What is a domain name?
In general, a domain name represents an Internet Protocol (IP) resource, such as a personal computer used to access the Internet, a server computer hosting a web site, or the web site itself or any other service communicated via the Internet.
According to the protocol introduced above, any Internet communication participant needs an IP address, e.g. 188.8.131.52. If you type it in the address bar of your browser, the google main page would be presented to you. However, you do not typically need to enter an IP address to access certain web resource, but only its domain name - http://google.bg, http://google.de, etc.
»Domain names organization
Domain names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including
- generic Top-Level Domains (gTLDs), such as com, net, org, etc.
- country code generic Top-Level Domains (ccTLDs), such as bg, de, etc.
Second-level and third-level domain names are typically open for reservation by end-users who wish to connect local area networks to the Internet, create other publicly accessible Internet resources or run web sites.
More on that matter: en.Wikipedia.org/wiki/Domain_name