Screens are measured diagonally, usually in inches. The most common sizes at the moment vary between 19" and 24".

There are two shapes of screen: normal (4:3 ratio) and widescreen (16:9 ratio). Widescreen is the current standard.

If the height of the screen is important (e.g. you need to see a whole page of text or to see as much of a spreadsheet as possible), then you might be better choosing a standard (4:3 ratio) screen.

Connecting to your screen - there are four common methods of connection.

VGA - the most common - a 15-pin analogue connection

DVI - these connections can be analogue (DVI-I) or digital (DVI-D)

HDMI - these digital connections are standard on most laptops or TV/entertainment equipment

mini-HDMI - these smaller connections are often found on small laptops and tablets

Connectors are available to convert analogue VGA to DVI-I, and digital DVI-D to HDMI.

DisplayPort (and Mini-DisplayPort) - this is the newest type of video connection type.

(Apple calls the Mini-DisplayPort "Thunderbolt".)

Connectors to convert to other types of video output are readily available.