What is that film making piece of equipment called?

I've seen directors and cinematographers carry a viewing lens around their necks. They use it to frame up a shot or get an idea of what will be on camera before they film. What is that thing called? If I wanted to buy one, do I need to be concerned about whether or not it matches the camera I will use?|||It's called a director's viewfinder: http://www.directorsviewfinders.com/