Klystron is a Microwave Generator

Compare SLAC's large, high-power microwave generator (klystron - below) with this much smaller one (magnetron - right) from a typical microwave oven.

A klystron looks and works something like an organ pipe.
Organ Pipe	In an organ pipe: Blowing into the organ pipe produces a flow of air. Flowing air excites vibrations in the cavity of the whistle. The vibrations flow into the surrounding air as sound waves.

In a klystron: The electron gun produces a flow of electrons. The bunching cavities regulate the speed of the electrons so that they arrive in bunches at the output cavity. The bunches of electrons excite microwaves in the output cavity of the klystron. The microwaves flow into the waveguide , which transports them to the accelerator. The electrons are absorbed in the beam stop.    Source:http://www2.slac.stanford.edu/vvc/accelerators/klystron.html