When a domain name expires, there's a time period of a little over sixty days when it's inactive, but can still be renewed by its current owner. During this period, the domain cannot be taken by anybody else, but if it is not renewed, it gets into the so-called Pending Deletion period that lasts five days. This is when ICANN-accredited registrar companies accept backorders for this kind of domains. If a given name is backordered, the registrar will make an attempt to register it for the customer immediately after it's deleted and offered to the general public, which gives people an opportunity to obtain a short and easy to remember domain which may have been used by another person up to now. If the domain is very attractive, a number of companies can 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 particular person will get that domain name. Nonetheless, using a backorder service will give you a lot better chance to get a particular domain than when you attempt to register it yourself.