A cathode ray tube (CRT), in the context vintage computers, usually refers to the glass part of a traditional computer monitor. The term is also used to distinguish traditional displays from modern [wiki]LCD[/wiki] monitors. CRTs are also used for television screens or [wiki]oscilloscope[/wiki] displays.
Theory of operation

The CRT itself is a vacuum tube, i.e. most of the air present during construction is removed during the manufacturing process. The face of the CRT, on which the image is displayed, is coated on the inside with a flourescent material (usually phosphor). This material is made to glow by means of one or more electron guns at the neck of the CRT (the opposite end of the tube). The stream of electrons inside the tube is deflected by means of electromagnetism to effectively trace the image on the display.

The circuitry required to drive a CRT produces very high voltages, in the range of several kilovolts (thousands of volts). The CRT itself also acts as a [wiki]capacitor[/wiki], storing a significant charge even after the unit is powered off. For these reasons caution is necessary when working on CRT equipment such as vintage computer monitors. If at all uncertain, consult a qualified technician before commencing work on such equipment. The primary components of a CRT include the fly back transformer with up to several thousand volts of electricity.
Examples of common CRT displays

[wiki]IBM 5151[/wiki] (Black and White monitor which came with the original [wiki]IBM PC (5150)[/wiki] and the [wiki]IBM PC XT (5160)[/wiki] and [wiki]IBM PC AT (5170)[/wiki]

[wiki]IBM 5153[/wiki] (RGBI color monitor which came with computers such as the [wiki]IBM PCjr (4860)[/wiki]