When a domain expires, there's a period of a little over 2 months when it's inactive, but can still be renewed by the current user. During this time period, the domain cannot be taken by anybody else, but in case it is not renewed, it goes into the so-called Pending Deletion period which lasts 5 days. This is the time when ICANN-accredited registrar companies accept backorders for this type of domains. If a given name is backordered, the registrar will try 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 memorable domain that may have been used by someone else until now. If the domain is really appealing, a couple of companies can try to register it in the same time and due to the fact that this happens on a first-come, first-serve basis, there's no warranty that a particular person will have that domain. Nevertheless, employing a backorder service will give you a much better chance to acquire a particular domain than if you attempt to register it personally.