When a domain expires, there is a time period of a little over 60 days when it's not active, but can still be renewed by the current owner. During this time, the domain cannot be taken by anybody else, but if it isn't renewed, it enters the so-called Pending Deletion period which lasts five days. This is the time when ICANN-accredited registrar firms take backorders for this kind of domains. When a given name is backordered, the registrar will make an attempt to register it for the customer the instant it is deleted and offered to the public, which gives people an opportunity to acquire a short and easy to remember domain that may have been used by someone else up to now. If the domain is quite attractive, a number 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 specific person will get that domain. However, using a backorder service will give you a better chance to acquire a specific domain than if you attempt to register it manually.