When a domain name expires, there is a time period of a little over eight weeks when it is not active, but may still be renewed by the current owner. During this period, the domain cannot be taken by anybody else, but in case it isn't renewed, it gets into the so-called Pending Deletion period which lasts five days. This is when ICANN-accredited registrar firms take backorders for this kind of domain names. When a specific name is backordered, the registrar will attempt to register it for the customer right after it is deleted and offered to the public, which gives people the chance to acquire a short and easy to remember domain that may have been used by someone else up to now. In case the domain is very appealing, a couple of companies could try to register it in the same time and since this happens on a first-come, first-serve basis, there's no guarantee that a specific person will have that domain. Nevertheless, employing a backorder service will give you a lot better chance to obtain a particular domain than when you try to register it yourself.