Drop registrars
A drop registrar is a domain name registrar that exists solely to catch expiring internet domain names, for the purpose of selling them. Usually they work for a domain backorder service, and receive a percentage of the final auction price.
The major domain backorder services are: Pool.com, Namewinner.com, Snapnames.com and eNom Club Drop. Go Daddy also offers domain backorders, but they do not use a network of drop registrars (all of their registrars are owned by the Go Daddy Group, Inc.).
A nearly complete list with information on each registrar that catches expiring domains is available at Whois-Search.com. Note that a registrar is not considered a drop registrar if they also sell domains retail or through resellers.
There are many services that maintain databases of domains names that have expired each day: ExpiredDomains.com, DomainsBot.com, DotCenter.com, Whois.sc, and Droplist.at. More information about expired domains and dropcatchers can be found at Expired-Domains-Guide.com.
Domain expiration process
A domain name that has not been renewed by its expiration date goes through a set of procedures until it is officially dropped from the ICANN database.
First, the domain enters "expired" status for 40 days. During this period, the owner of the domain is still able to renew the domain for a nominal domain renewal fee.
After the 40-day "expired" period is over, the domain goes into "redemption" period, during which its whois record starts to be removed, and the domain owner needs to pay an additional fee to renew the domain.
Finally, the status of the domain changes to "locked" status as it enters the deletion stage, which normally lasts for five days. On the fifth day, the name is officially removed from the ICANN database and becomes avaialble to the public for registration.
