One might be referring to a broker who lists bank owned properties when they use this term. Foreclosure is a process. When a property is in foreclosure, the current owner retains title and rights. When a property is foreclosed, the process is complete and the lender takes back the ownership of the property. It is foreclosed, and becomes an REO property.
There isn't really such thing as a foreclosure broker......although some agents and brokers are more familiar w/ the foreclosure process than others. If a seller is facing foreclosure and trying to secure a sales contract and title transfer before a foreclosure action is complete, it may be helpful to hire an agent who has some knowledge of the process.
There are a few brokers who have established relationships with the REO depts at banks and list the REO properties for the banks. One might call these brokers, "foreclosure brokers".....but it's not an official term.