An X-ray tube is a vacuum tube that converts electrical energy into X-rays, which are used in medical imaging, dentistry, and research: 
 
X-ray tube
How it works
Electrons are accelerated from a cathode to a target anode, producing X-rays
Parts
Cathode, anode, glass envelope
Purpose
Produces a controllable source of X-rays for imaging and therapy
Applications
Medical imaging, dentistry, research, and exploring crystalline materials
Here's how X-ray tubes work:
Cathode: A heated filament that produces electrons through thermionic emission 
Anode: A solid metal target that converts the electrons' energy into X-rays 
Vacuum: Prevents electron dispersion and ionization, which can damage the filament 
Voltage: A high-voltage field accelerates the electrons to high speeds 
X-ray production: Only about 1% of the electron energy is converted to X-rays 
X-ray tubes are designed to minimize heat production and maximize X-ray output. The American engineer William D. Coolidge invented the X-ray tube design in 1913.