When a domain expires, there is a time period of a little over eight weeks when it's not active, but may still be renewed by the current owner. During this time, the domain cannot be taken by anybody else, but if it isn't renewed, it enters the so-called Pending Deletion period that lasts 5 days. This is when ICANN-accredited registrar firms accept backorders for this kind of domains. When a specific name is backordered, the registrar will make an attempt to register it for the customer immediately after it is deleted and accessible to the general public, which gives people the option to get a short and memorable domain which may have been used by some other person up until this moment. In the event that the domain is extremely appealing, several companies could 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 particular person will get that domain. Nonetheless, using a backorder service will give you a lot better chance to acquire a specific domain than if you try to register it yourself.