When a domain expires, there's a period of a little over sixty days when it's inactive, but could still be renewed by the current user. During this time period, the domain cannot be taken by anybody else, but if it isn't renewed, it gets into the so-called Pending Deletion period which lasts five days. This is when ICANN-accredited registrar companies accept backorders for this type of domains. If a given name is backordered, the registrar will attempt to register it for the customer as soon as it is deleted and offered to the general public, which gives people a chance to obtain a short and memorable domain which may have been used by somebody else up until this moment. If the domain is quite appealing, several companies could 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 have that domain name. However, using a backorder service will give you a better chance to end up getting a particular domain than when you attempt to register it personally.
