A display is an output device that takes a signal from a system and converts it into an optical display of some sort.  These devices are designed commonly as both analog and digital displays, and the technology and design of output optical devices have grown by leaps and bounds in the late 20th and early 21st century.  They can be used to relay simple character information or as a complex graphical and color-complex unit.  They are most commonly rated according to refresh rate (how fast data can change on the screen), contrast, power draw, brightness, and lifespan.

Simple LCD displays have been around for decades, and can be seen in common calculators or printer outputs.  More complex LCD and LED technologies are omnipresent now as computer output displays and probably enjoy the broadest usage of any other display technology at the moment.  OLED is an upcoming technology that may be the next breakthrough, and it could allow displays to be as skinny as a few sheets of paper.