It sounds like you're talking about the kind of rail system that's used for mounting video equipment. There's some detailed information [here][1] and a [video here][2] but in a nutshell a rail system is often the foundation upon which all kinds of video equipment is mounted. That includes the camera, of course, but also sound equipment, [follow focus adjuster][3], [matte box][4], additional displays and interfaces, lights, batteries, [shoulder mount][5], tripod mount, perhaps even a [teleprompter][6]. 


  [1]: http://cameradojo.com/2009/10/07/what-is-a-rail-system/
  [2]: https://www.youtube.com/watch?v=Gdt4QJ29s6Q
  [3]: http://ikancorp.com/productdetail.php?id=310
  [4]: http://store.redrockmicro.com/MatteBox.html
  [5]: http://store.redrockmicro.com/Catalog/microShoulderMountRigs/microshoulderMountDeluxe.html
  [6]: http://www.autocue.com/news/050913-autocue-launch-dslr-prompter-at-ibc