This is coming from a discusion of "Theory" in week four of PLENK2010
A theory is a simplified model of some real world phenomena which tries to encode previous experience with a view to guiding future action.
A really good theory will make wild improbable predictions which turn out to be true. Think things which glow in the dark and the photoelectric effect leading to H bombs here.
A fun theory starts with minute observations of some trivial inconsistency in conventional explanation and replaces it with a much grander structure.
Many theories will cover the facts, few give you a totaly new vision of the existing landscape.
I would sugest
The school of Barbianna
Foucault's analysis of power in the classroom
as examples of theories which do.