In an LCD, tiny packets of liquid crystal make up each pixel. These don’t actually radiate any light. Instead they change their structure when hit by voltage, so they need a backlight (cold-cathode or LED). In OLED, each pixel is an array of itsy-bitsy colored LEDs that glows in corresponding color initiated by the TV signal.
You can view OLED screens from any angle and they can show a wider range of colors (and black areas are a deeper black). Because they don’t need a backlight they can be much thinner while actually consuming less electricity.
Read the Reuters story here.