There are two kinds of displays used in alarms, digital and analogue. The first kind of displays were analogue yet more modern clocks have digital formats which either use an LED format or by using Liquid Crystal Displays, or LCD in short.
The original type of clock display is the analogue display, this typically has clock hands and numbers to show the time, it will have an hour hand and a minutes hand, and
some alarms have a seconds hand to show the most up to date time. An example analogue display is shown below.
Analogue display clocks are typically battery powered or use the old traditional wind up clock mechanism.
More modern clock designs have digital displays which are either shown by liquid display crystal or by using light emitting diodes (LED). The displays are all digital and can
come in some amazing designs and attractive colours.
Examples of digital display used in alarms include projections, ipod radio, radio controlled devices. LCD displays need a backlight at night to see the time whereas LED clock displays can be seen easily throughout the night, virtually all LED alarms are mains powered as they draw more power than the typical LCD device.
One of the more popular modern alarms today are projection clocks which project a digital display of the time against a wall or on the ceiling.