When a domain name expires, there is a time period of a little over sixty days when it's inactive, but can still be renewed by the current user. During this period, the domain cannot be taken by anybody else, but in case it's not renewed, it goes into the so-called Pending Deletion period that lasts 5 days. This is when ICANN-accredited registrar companies take backorders for such domains. When a particular name is backordered, the registrar will try to register it for the customer the second it's deleted and available to the public, which gives people the chance to obtain a short and easy to remember domain that may have been employed by somebody else until now. If the domain is very attractive, several 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 have that domain name. Nonetheless, employing a backorder service will give you a lot better chance to end up getting a particular domain than when you attempt to register it manually.