properties that are in the process of being foreclosed upon, or have been foreclosed on, but may not be listed for sale yet. When the bank re-sells these properties, they will most likely list them with a Realtor.
The ones that you see listed in Trulia are in various stages of the foreclosure process. The situation will either be resolved between the owner and the bank, it could be listed as a short sale where the bank accepts less than they are owed, or it'll go to the auction process where it'll be sold to the highest bidder subject to any superior liens that exist. Many times once it's at the auction, the bank will buy it back then it'll be listed as an REO as an as-is property.
Feel free to contact me directly with any other questions regarding your search.