differences between theoretical research and applied research
Rule of thumb:


Theoretical research concerns itself with models: defining models, determining their properties and theorems that hold given a set of models.
Applied research concerns itself with the real world: solving problems given a real context governed by physical (or social) laws.


Obviously, the two are intertwined and not always clearly separated. For instance, TCSists sometimes call a piece of work "applied" if the model of choice somewhat resembles reality, in contrast to being completely academic.