OLED stands for organic light-emitting diode.
Its panels are made from organic materials that emit light when electricity is applied to them.
As a result, OLED panels don't use a backlight meaning they are thinner than LCD displays.
OLEDs additionally have bright colours, brilliant contrasts and a wide-viewing angle.
The top manufacturers are banking on this new technology to take off and be the next big step, replacing LCD, Plasma and older LED technology.
To expand on this:
Each OLED pixel is its own light source (emissive display - like the old tube sets). LCD has a back light (the LED in current form, vs a very large fluorescent flat tube) and each pixel has a liquid crystal shutter. That means when the OLED pixel goes black, there is no light coming from it; all LCDs will have some amount of bleed through of light. Hence, much better contrast in OLED, and true blacks.
Also, turning a pixel on/off (or varying the brightness) is always going to be faster with OLED since you are just manipulating the flow of electrons. In LCDs, one manipulates the flow of electrons at the pixel to create a field, which then aligns the LCs to allow more/less light through. Thus, better motion rendering.
All that said, LCD TVs today are pretty damn good, especially the 4K. The high end 4K ones are probably on par (at least to the human eye) with OLED.
I bought a 60" Sony last summer off of Amazon for $700 and it's great. If you've got the scratch (~$4K I think), OLED. Otherwise any Sony, Samsung, LG is going to be very good.