A diaphragm (also referred to as a mouth call or reed) fits inside your upper palate. It can be used for cow sounds and bull sounds. In order to make bull sounds they are usually used with an amplifying tube (grunt tube, vacuum hose, etc..).
I also have custom hand turned calls that use an external reed for cow calls (also referred to as open reed). These use a mylar reed, sloped tone board with an air channel and a barrel.