A cathode ray tube works by sending electrons from the negatively charged cathode to the positively charge anode, which has a small tube to allow electrons to exit out in a focused line onto a screen. The screen is coated in some material, usually phosphor, which creates a temporary impression once struck with an electron.
Cathode ray tubes were often used in older models of television sets. They were typically set up with multiple tubes, or "guns," that pointed to green, red and blue pixels on a screen. The electrons would be funneled to the correct color using some sort of deflection method, usually magnetic.