When a domain expires, there's a time period of a little over two months when it is not active, but could still be renewed by its current owner. During this time period, the domain cannot be taken by anybody else, but in case it isn't renewed, it enters the so-called Pending Deletion period which lasts five days. This is when ICANN-accredited registrar firms take backorders for this kind of domain names. If a particular name is backordered, the registrar will make an attempt to register it for the customer immediately after it's deleted and offered to the public, which gives people the opportunity to acquire a short and easy to remember domain which may have been used by some other person up to now. If the domain is really appealing, a number of companies may try to register it in the same time and since this happens on a first-come, first-serve basis, there's no guarantee that a particular person will get that domain name. However, employing a backorder service will give you a better chance to end up getting a particular domain than when you try to register it personally.