The cavity magnetron
Invented at Birmingham University in 1940 by Boot and Randall. A copper block with resonant cavities, with a hot cathode at the centre and a magnetic field perpendicular to the electric field. Electrons spiral and dump energy into the cavities, producing tens of kilowatts at 10 GHz from a device the size of a soup can. It made airborne radar practical overnight.
Why magnetrons are dirty
A magnetron oscillates freely — the frequency drifts with temperature and load. You can't make it coherent pulse-to-pulse, which kills Doppler processing. It's perfect for cheap weather radar and microwave ovens (same physics, lower power), useless for modern MTI or pulse compression.
The klystron's precision
A klystron is a velocity-modulated amplifier. An input signal modulates an electron beam, the beam drifts and bunches, and a resonant cavity extracts amplified output. The output is phase-coherent with the input — you can pulse-compress, Doppler-process and integrate coherently. Klystrons power most ground-based long-range radars and particle accelerators (SLAC ran on klystrons for decades).
Still alive
Solid-state has replaced both in many roles, but the power density of a tube at high frequencies is still unmatched. NEXRAD weather radars use klystrons. Marine radars use magnetrons because they're cheap and don't need coherence. The microwave oven in your kitchen is still a 1940s magnetron.