When a domain expires, there's a time period of a little over eight weeks when it's not active, but can still be renewed by its current user. During this period, the domain cannot be taken by anybody else, but in case it is not renewed, it gets into the so-called Pending Deletion period that lasts 5 days. This is the time when ICANN-accredited registrar firms take backorders for such domains. When a specific name is backordered, the registrar will attempt to register it for the customer right after it's deleted and offered to the general public, which gives people the opportunity to obtain a short and memorable domain that may have been used by someone else up to now. If the domain is very attractive, 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 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 if you attempt to register it personally.