When a domain name expires, there's a period of a little over 2 months when it's not active, but can still be renewed by its current owner. During this time period, the domain cannot be taken by anybody else, but if it isn't renewed, it gets into the so-called Pending Deletion period which lasts 5 days. This is the time when ICANN-accredited registrar companies take backorders for such domain names. In case a given name is backordered, the registrar will attempt to register it for the customer the second it's deleted and accessible to the general public, which gives people an opportunity to get a short and easy to remember domain which may have been used by somebody else until now. If the domain is rather attractive, a couple of companies may try to register it in the same time and since this happens on a first-come, first-serve basis, there is no guarantee that a particular person will get that domain name. Nevertheless, using a backorder service will give you a much better chance to obtain a particular domain than if you try to register it personally.