A Light-Emitting Diode is an electronic lamp device, used for indicator and numeric displays (seven-segment displays). Some cameras (such as the Canon A-1) use seven-segment numeric displays in their viewfinders. Originally, LEDs were all red; a little later, green was available, and then yellow; only quite recently have blue and - very recently, white - LEDs been possible.
An Organic Light-Emitting Diode uses electroluminescent compounds which are organic (carbon-based) chemicals. Different chemistries permit these LEDs to shine with different colors of light. An Active Matrix OLED (AMOLED) display may include millions of individually-addressed OLEDs. The technology is increasingly popular for digital camera EVFs and rear-panel displays.