HQueue, or Houdini Queue, is a distributed job scheduling system. It consists of an HQueue server and a farm of processing nodes, or clients. The server accepts job submissions from users and assigns them to available clients. The HQueue Render output driver submits render jobs to HQueue for distribution on the farm. The actual rendering is done by the node specified in the Output Driver parameter. This is typically a Mantra ROP, but can also be a Composite ROP or other output driver.
Source: HQueue Render