Computer connected to the Internet are using the Internet Protocol (IP) for communication. Each computer has at least one unique IP-address to identify itself.
An IP-address consists of numbers which is not very easy to remember and use for humans. To make an address more user friendly a domain name is mapped to an IP-address.
Domains follow a hierarchical order.
A domain name consists of domains which are seperated by dots. The domain which is the most right (Top Level Domain) is the hierarchical highest. Every domain is a subdomain of its right domain (i.e. a.b.c is a subdomain of b.c, and b.c is a subdomain of c).
A domain is caseinsensitiv. A domain's maximum length is 63 characters a domain name's is 255 characters (dots included).