This IP address is translated into a human friendly name which is called a domain name. This website www.expiredurl.info for example is the friendly name.
Domain names are the same as any other property or item in that the better the word or more popular the word, the more valuable it is. Hot names like iphone and apple are worth millions of dollars simply because they are guaranteed to be busy websites. Acquiring domains such as these is virtually impossible, however on occasion, it is possible. That is where the term expired domains comes into play. A domain is purchased and registered, either annually, bi-annually, every 5 or ten years. Occasionally, the registrant may forget to renew a domain name, they may no longer need the domain, or maybe their company has gone out of business. For whatever reasons, domain names expired of the renewal fees are not paid. There is a short grace period to recover the domain beyond it's expiry date, and then it becomes openly available.
This is a huge business, and often very profitable, however it is also potentially unethical - in the case of a domain being purchased for the purpose of cyber-squatting. Cyber-squatting is a common occurrence when a popular domain is purchased and then resold at a huge profit to the original owner. The resale may be consider a form of extortion, however the bottom line is that if you failed to renew the domain it is publicly available.
Newly expired domains are available everyday and can be found in daily lists. Many people utilize these lists to purchase these expired domains and either develop them into websites, or simply resell the domain name. expiredurl.info for example is a source of these domain lists, as well as a source to generate 3 character domains - which are also very popular. The shorter the domain, the easier to remember. Expired url offers a domain name search and domain name generator utility as well.