When a domain expires, there is a period of a little over sixty days when it is inactive, but can still be renewed by its current user. During this time period, the domain cannot be taken by anybody else, but if it is not renewed, it gets into the so-called Pending Deletion period which lasts 5 days. This is the time when ICANN-accredited registrar firms accept backorders for this kind of domains. If a particular name is backordered, the registrar will make an attempt to register it for the customer the moment it is deleted and offered to the general public, which gives people the option to get a short and memorable domain that may have been employed by someone else up until this moment. If the domain is rather appealing, a number 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 guarantee that a specific person will have that domain. Nonetheless, employing a backorder service will give you a better chance to obtain a particular domain than if you attempt to register it manually.