When a domain name expires, there's a period of a little over 2 months when it's inactive, but could still be renewed by its current owner. During this time, the domain cannot be taken by anybody else, but if it isn't renewed, it goes into the so-called Pending Deletion period which lasts five days. This is when ICANN-accredited registrar companies take backorders for such domain names. When a given name is backordered, the registrar will try to register it for the customer right after it is deleted and available to the public, which gives people an opportunity to obtain a short and memorable domain that may have been used by someone else until now. In case the domain is quite appealing, a couple of companies can try to register it in the same time and because this happens on a first-come, first-serve basis, there is no warranty that a particular person will get that domain. However, employing a backorder service will give you a better chance to get a specific domain than if you attempt to register it manually.