When a domain expires, there is a time period of a little over two months when it is not active, but can still be renewed by its current user. During this time period, the domain cannot be taken by anybody else, but if it isn't renewed, it enters the so-called Pending Deletion period which lasts 5 days. This is when ICANN-accredited registrar companies take backorders for these domains. If a given name is backordered, the registrar will make an attempt to register it for the customer the instant it is deleted and available to the general public, which gives people the chance to acquire a short and easy to remember domain which may have been used by somebody else up until this moment. In the event that the domain is quite attractive, 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 is no guarantee that a specific person will get that domain name. Nonetheless, employing a backorder service will give you a much better chance to get a specific domain than if you try to register it yourself.