When a domain name expires, there's a time period of a little over 60 days when it's not active, but can still be renewed by its current owner. During this time, the domain cannot be taken by anybody else, but in case it's not renewed, it enters the so-called Pending Deletion period that lasts five days. This is the time when ICANN-accredited registrar companies take backorders for such domains. In case a given name is backordered, the registrar will make an attempt to register it for the customer the second it's deleted and accessible to the general public, which gives people the opportunity to obtain a short and memorable domain which may have been employed by someone else up until this moment. In case the domain is extremely appealing, a couple of companies could 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 warranty that a particular person will have that domain name. Nonetheless, using a backorder service will give you a lot better chance to obtain a specific domain than when you try to register it manually.