Cathode ray tubes (CRTs) are vacuum tubes that use electron beams to display images on a fluorescent screen. They contain an electron gun that emits electrons which are accelerated and deflected by magnetic coils to scan across the screen. Pixels are created as the beams excite red, green, and blue phosphor dots. CRTs were commonly used in monitors and televisions but have disadvantages like large size, potential health risks from radiation, and heavy weight. Input devices like keyboards, mice, and scanners are used to enter data into computers, while output is displayed on monitors or printed with printers.