When a domain name expires, there's a time period of a little over 60 days when it's inactive, but can still be renewed by its current owner. During this time period, the domain cannot be taken by anybody else, but in case it's not renewed, it gets into the so-called Pending Deletion period which lasts five days. This is the time when ICANN-accredited registrar companies accept backorders for such domains. If a particular name is backordered, the registrar will attempt to register it for the customer immediately after it's deleted and available to the public, which gives people a chance to obtain a short and memorable domain that may have been employed by someone else up until this moment. In the event that the domain is quite appealing, a number of companies may try to register it in the same time and because this happens on a first-come, first-serve basis, there is no guarantee that a specific person will get that domain name. Nonetheless, using a backorder service will give you a lot better chance to get a particular domain than if you try to register it yourself.