When a domain expires, there is a period of a little over 60 days when it's inactive, but may still be renewed by its current user. During this period, the domain cannot be taken by anybody else, but if it's not renewed, it gets into the so-called Pending Deletion period which lasts five days. This is when ICANN-accredited registrar companies take backorders for such domains. In case a specific name is backordered, the registrar will make an attempt to register it for the customer as soon as it's deleted and accessible to the general public, which gives people the option to get a short and easy to remember domain which may have been used by another person up to now. In the event that the domain is quite appealing, a couple of companies can 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. Nevertheless, using a backorder service will give you a lot better chance to get a particular domain than when you try to register it yourself.