When a domain expires, there's a period of a little over 2 months when it is inactive, but may still be renewed by the current owner. During this time, the domain cannot be taken by anybody else, but if it's not renewed, it goes into the so-called Pending Deletion period that lasts five days. This is when ICANN-accredited registrar firms accept backorders for these domains. If a given name is backordered, the registrar will try to register it for the customer as soon as it is deleted and offered to the public, which gives people the chance to acquire a short and easy to remember domain that may have been used by some other person up to now. In the event that the domain is quite appealing, several companies could try to register it in the same time and because this happens on a first-come, first-serve basis, there's no warranty that a particular person will have that domain name. Nonetheless, using a backorder service will give you a lot better chance to get a specific domain than if you attempt to register it personally.