A domain name is your address on the Internet. It is linked to a numerical address called an IP address (e.g. 209.2 07.176.96). A domain name server, commonly referred to as "DNS" is used to translate a domain name into an IP address.
Domain names consist of two parts: the actual site name, and the top level domain to which it belongs. The top level domains available in the US for public use are .com, .net, and .org.