Liquid crystal display
Liquid crystal displays are often used in battery-powered devices, such as digital watches, because they use very little electricity.  They are also used for flat screen TV's.  Many LCDs work well by themselves when there is other light around (like in a lit room, or outside in daylight).  For smartphones, computer monitor, TV's and some other purposes, a back-light is built into the product. This is  an LED or Cold Cathode Fluorescent Light (CCFL).