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 user. During this period, the domain cannot be taken by anybody else, but in case it isn't renewed, it enters the so-called Pending Deletion period that lasts five days. This is when ICANN-accredited registrar firms accept backorders for this kind of domains. If a particular name is backordered, the registrar will attempt to register it for the customer as soon as it's deleted and accessible to the public, which gives people an opportunity to acquire a short and memorable domain that may have been employed by someone else up to now. In the event that the domain is quite attractive, a number of companies can try to register it in the same time and due to the fact that this happens on a first-come, first-serve basis, there is no guarantee that a particular person will get that domain. Nonetheless, employing a backorder service will give you a lot better chance to acquire a specific domain than if you try to register it manually.