Monitor is a display device connected to the display adapter of a computer. The analog CRT displays have usually only a VGA connector, but the digital LCD/TFT displays are equipped with a DVI connector and may also have a normal VGA connector in addition.
The most important measures of a monitor are the screen size, maximum resolution and the refresh rate. The screen size is measured in inches (eg 17"). Resolution is defined as pixels by pixels (1024 * 768) and refresh rate as hertz (85Hz). Pixel size may also be of importance.
A monitor is not mandatory for a computer to operate, eg. servers are sometimes run without a monitor and administered by connecting remotedly.