When a domain name expires, there is a time period of a little over two months when it's not active, but may still be renewed by the current owner. During this time period, the domain cannot be taken by anybody else, but in case it's not renewed, it enters the so-called Pending Deletion period which lasts 5 days. This is when ICANN-accredited registrar companies take backorders for this kind of domains. If a specific name is backordered, the registrar will make an attempt to register it for the customer the instant it's deleted and available to the general public, which gives people the option to obtain a short and easy to remember domain that may have been employed by someone else until now. In case the domain is very appealing, a number 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's no warranty that a particular person will have that domain name. Nevertheless, using a backorder service will give you a much better chance to obtain a particular domain than when you try to register it manually.