When a domain expires, there is a time period of a little over two months when it is inactive, but could still be renewed by its current owner. During this time period, the domain cannot be taken by anybody else, but if it's not renewed, it gets into the so-called Pending Deletion period that lasts five days. This is when ICANN-accredited registrar firms take backorders for such domain names. When a specific name is backordered, the registrar will make an attempt to register it for the customer the moment it is deleted and available to the general public, which gives people a chance to get a short and memorable domain which may have been used by somebody else until now. If the domain is rather appealing, a couple of companies can try to register it in the same time and because this happens on a first-come, first-serve basis, there's no warranty that a particular person will have that domain. Nonetheless, employing a backorder service will give you a better chance to obtain a particular domain than when you attempt to register it yourself.