I am assuming (based on Ingres and Oracle experience) that these workfiles were what SQL Server used for sorting and aggregate functions when they became too big to fit in physical memory.
The reason I pose the question is because I am evaluating a monitoring tool, which has worktables and workfiles in the same performance graph (assume they are related then?). I am happy that worktables are temp sort areas in tempdb, but do not know what these workfiles are? - which brings me back to my initial assumption.
I cannot find any ref to them in BOL but I have found them mentioned in MS Support Site - but no real detail.