When a domain expires, there is a time period of a little over two months when it's inactive, but can still be renewed by the current owner. During this time period, the domain cannot be taken by anybody else, but if it is not renewed, it enters the so-called Pending Deletion period that lasts five days. This is the time when ICANN-accredited registrar firms accept backorders for such domain names. If a particular name is backordered, the registrar will try to register it for the customer right after it's deleted and accessible to the general public, which gives people the option to obtain a short and memorable domain which may have been employed by some other person until now. In the event that the domain is rather appealing, several companies may try to register it in the same time and since this happens on a first-come, first-serve basis, there is no warranty that a particular person will have that domain name. However, employing a backorder service will give you a better chance to end up getting a particular domain than if you attempt to register it personally.