Images produced by a computer or another electrical instrument are displayed on a VDU. Although "monitor" and "VDU" are frequently used interchangeably, the word can also apply to other displays, like digital projectors. Video projection units may be incorporated with the other components or they may be auxiliary devices. A good illustration of an all-in-one design is the Apple iMac, which combines the computer and screen into a separate item.

The majority of early VDU screens were cathode ray tubes (CRTs), and their diagonal sizes were usually 13 inches or smaller. In the 1990s, 15" and 17" screens became the norm, and some producers started making displays larger than 20". CRT displays were difficult to obtain by 2006 as flat panel displays started to proliferate at the turn of the century.

These days, laptops frequently come with VDUs that range in size from 20" to 30". The latest advancements in LCD, plasma, and LED technology have made it significantly more affordable than ever before to manufacture big displays.