What does a rim shader do and what is it used for? In what cases would it make sense to utilize a rim shader?
In the context of graphics programming and shaders, a "rim" shader would be one that focuses on achieving a lighting effect related to the silhouette or other feature/contour edges of an object (crudely, "the outline" of the object). Typically this would be to simulate rim lighting (more examples), which comes about from back-lighting a subject.
I suppose if you wanted to stretch it, you could consider shaders employ other edge-related effects (such as drawing the outlines on cel shaded scenes) "rim shaders" as well, since they do have something to do with the feature edges of the object. More I think more traditionally the term is used to refer to the lighting effect.