A Computer Monitor is the screen that displays the operating systems, and user interface. Today, it is common for CRT and LCD monitors to be used with PCs. A CRT monitor is a Cathode Ray Tube, they are big deep boxes much like many older TV sets. An LCD Monitor is a this flat screen and stands for Liquid Crystal Display.
Choosing the right monitor is important with games and specialty uses such as photo or video editing that requires high quality and large sized images. Note that LCD monitors are known as Flat Panel monitors, however, CRT monitors can be Flat Screen if the front glass is flat instead of curved.