A domain name is a combination of letters, numbers and some additional characters (usually words, phrases or accronyms) combined using full stops, with other character combinations (TLDs and Subdomains). The purpose of domain names is purely for human memory as words and phrases are far simpler to remember than the IP addresses they translate to.
Domain names are used in computer networks and application-specific naming and addressing. In general, a domain name represents an Internet Protocol (IP) resource, such as a personal computer used to access the Internet, a web hosting server, or the web site itself or any other service connected via the Internet.
Domain names are formed by the rules of DNS (Domain Name System). DNS servers are used to convert a a domain name to an IP address. DNS servers are a bit like a directory of names and numbers, such as telephone directories. A DNS server will look up the domain name in its database and return an IP address (Just like looking for somebodies telephone number by using their name and surname). When a DNS server doesnt have a domain name in its database it makes a lookup to a parent or root DNS server (nameserver). The root servers have records of the child nameservers which are capable of converting the domain name in question to an IP address.
Domain names are organized in levels (subdomains) of the root DNS domain. The first-level of domain names are top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, net and org, and the country code top-level domains (ccTLDs). The TLD forms the last part of a domain name.
Below the top-level domain in the hierarchy is second-level and third-level domains which are purchased by businesses and individuals through domain registrars.
Finally, there are subdomains which often form part of the full of domain name.
www = The Subdomain
theserve = The second-level domain
co.uk = The TLD (A ccTLD)