What is an electron gun?

It is a device in the back of a "cathode ray tube" television or computer monitor (pre-plasma or LCD flat-panel). The electron gun fires a stream of electrons from the back of the TV onto the inside face of the screen, which is coated with a material that glows when hit by electrons. By using powerful magnets to direct the stream across the screen, a series of dots ('pixels') is built over the screen which appears as an image.