Data mining and, more specifically, text mining are research techniques, using computational analysis to uncover patterns in large data sets. "Text mining" is just data mining in large text-based data sets.
This kind of analytic tool is useful in numerous scholarly fields, from the humanities (where it is sometimes viewed as one of the tools of "digital humanities" scholars), to the sciences, where useful data can be "mined" from large non-text datasets and from text databases of the published literature.