What is waterjet cutting?

Waterjet cutting machines use a high pressure ( 50,000 psi ) jet of water less than 1mm wide to cut through virtually any material.
Depending on the machine the material can be up to 125mm thick.
Depending on the material being cut an abrasive is mixed in with the water as it passes through the cutting jet.

What sort of machine is used?

These days most machines use CNC ( computer numerical control ) to control the cutting jet.
This allows extremely precise control of the cutting jet, which in turn means greater accuracy in the finished part.
Virtually any 2d shape can be cut in an endless variety of materials.