When a domain name expires, there's a period of a little over sixty days when it is inactive, but can still be renewed by its current user. During this period, the domain cannot be taken by anybody else, but in case it's not renewed, it enters the so-called Pending Deletion period which lasts 5 days. This is when ICANN-accredited registrar firms accept backorders for this kind of domain names. In case a given name is backordered, the registrar will attempt to register it for the customer as soon as it is deleted and accessible to the public, which gives people an opportunity to obtain a short and memorable domain that may have been used by somebody else until now. In case the domain is very appealing, a couple of companies could 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 specific person will have that domain. However, using a backorder service will give you a much better chance to acquire a specific domain than when you attempt to register it manually.