When a domain name expires, there's a time period of a little over sixty days when it is inactive, but can still be renewed by its current user. During this period, the domain cannot be taken by anybody else, but in case it's not renewed, it enters the so-called Pending Deletion period which lasts 5 days. This is the time when ICANN-accredited registrar firms accept backorders for this kind of domains. When a given name is backordered, the registrar will try to register it for the customer the moment it's deleted and offered to the general public, which gives people the chance to obtain a short and easy to remember domain that may have been employed by someone else until now. In the event that the domain is extremely attractive, a number of companies can try to register it in the same time and because this happens on a first-come, first-serve basis, there's no guarantee that a specific person will have that domain. Nevertheless, employing a backorder service will give you a better chance to end up getting a particular domain than when you attempt to register it yourself.