When a domain name expires, there is a time period of a little over 60 days when it's not active, but could still be renewed by the current user. During this period, the domain cannot be taken by anybody else, but in case it's not renewed, it goes into the so-called Pending Deletion period which lasts five days. This is the time when ICANN-accredited registrar companies accept backorders for this type of domain names. If a specific name is backordered, the registrar will make an attempt to register it for the customer the instant it is deleted and accessible to the general public, which gives people an opportunity to acquire a short and memorable domain which may have been used by somebody else until now. If the domain is really appealing, several 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 have that domain. Nonetheless, using a backorder service will give you a better chance to end up getting a specific domain than if you attempt to register it yourself.