A domain name name is a name given to an IP address such as www.yahoo.com. A domain name name is much like knowing a house by "Bob's" instead of his exact street address. Domain names provide an easy way to get to a website instead of remembering a long IP address.
Hostnames are restricted to using only the ASCII letters "a" through "z' (case-insenstive), the digits "0" through "9" and the hyphen, along with a few other restrictions.
Every domain name ends in a top-level domain (TLD) name, which is always either one of a small list of generic names (three or more characters), or a two characters territory code. Top-level domains are sometimes also called first-level domains. .com, .net, .us, .cn, .ca are examples of top-level domains.