1) pending delete auctions. These are domains that were not renewed and are going through a pretty complicated drop process. You can backorder these names at the auction venues and then each attempts to "catch" the dropping domain. If one is successful you win the domain at $59 if you're the only one to backorder the name. If more than one person backordered the domain a 3 day private auction is held. Once the domain goes through the drop process, the back links have pretty much zero SEO benifit to the new domain owner.
Thousands of domain names expire every day. The reasons are different. Some owners forgot to renew the domains, some just don't want them anymore or they moved on to other projects. For most people these so called Expired Domains don't have any value. They just see a bunch of Domain Names someone else deleted and move on, but for the people who know about SEO or the value of good Backlinks, Expired Domain Names are money just waiting to get picked up from the street. The only problem is to separate the good ones from the bad ones. That is where ExpiredDomains.net comes into play.