Drop registrar

A drop registrar is a domain name registrar who registers expiring Internet domain names immediately after they expire and are deleted by the domain name registry. A drop registrar will typically use automated software to send up to 250 simultaneous domain name registration requests in an attempt to register the domain name first.[1] They usually work for a domain back-order service, and receive a percentage of the final auction price.

Note that a registrar is not considered a drop registrar if they also sell domains retail or through resellers. Individuals also try their hand at acquiring expiring domain names by paying for lists of domains that are going to expire in the very near future, unless the current owner decides to renew their registration with the registrar. There are sites that do this for free expiringlookup.com [1], the down side is that everyone will have the list as well.
http://en.wikipedia.org/wiki/Drop_registrar