Vue has 3 renderers: internal, the one that renders to screen, and 2 externals, one renders to disk, and the other renders on a network of computers. Rendercows are nodes you install on a this network of computers. The cows are monitored by the Hypervue manager , part of the Vue software.
Renderbull is another external renderer manager that integrates in a renderfarm or in another software render manager, like Backburner for 3ds Max. You will never use it unless you  work in a big studio.