When a domain expires, there's a time period of a little over sixty days when it is not active, but may still be renewed by its current user. During this time, 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 accept backorders for this kind of domains. When a given name is backordered, the registrar will attempt to register it for the customer immediately after it is deleted and offered to the general public, which gives people the option to obtain a short and easy to remember domain that may have been employed by somebody else up until this moment. If the domain is rather appealing, a couple of companies may try to register it in the same time and due to the fact that this happens on a first-come, first-serve basis, there is no guarantee that a particular person will have that domain. However, using a backorder service will give you a lot better chance to get a particular domain than when you attempt to register it manually.