Every pixel is its own light
An LCD shines one backlight through liquid crystals and color filters. An OLED panel has no backlight. Each subpixel is a tiny organic LED that lights itself, and a pixel that's off emits nothing. That's why OLED black is real black, and why contrast is limited by room light bouncing off the glass, not by the panel.
It also changes how dimming works. A 4K OLED screen has 8,294,400 pixels, and each one is its own dimming zone. A Mini LED screen has hundreds to thousands of zones, each covering many pixels. A bright object on a dark background can light up the zone around it, and OLED has no zone to leak into.
White OLED and QD-OLED
Two designs cover most monitors and TVs. WOLED makes white light and uses color filters to pick red, green and blue, and the filters absorb some of the light. QD-OLED uses a blue OLED layer with quantum dots that convert the blue into red and green, and it shows better color volume, meaning colors hold their saturation as they get brighter.
Both are OLED where it matters most: the pixels make their own light. The OLED vs QD-OLED page sets the two side by side.
Where OLED gives ground
Self lit pixels come with two limits.
The first is a power limit, called automatic brightness limiting (ABL). When the average picture gets bright, the screen dims itself to stay inside its power budget, and there's no setting that turns it off. A small highlight in a dark scene can reach a high peak, while a mostly white screen can't. It mostly shows up in HDR. Some screens also dim static content on purpose, which is a separate feature from ABL.
The second is wear. The organic material loses brightness with use, and blue emitters wear fastest, which is why makers build the blue subpixel larger. Uneven wear is what turns into OLED burn-in, and it's a real consideration for desk use with a fixed taskbar and windows.
Speed
OLED pixels can change in under 0.01 ms (10 microseconds), against about 1 ms for the fastest LCD transitions. That's at least 100 times quicker. It matters most at high refresh rates. At 480 Hz a new frame arrives every 2.08 ms, and a 1 ms LCD transition fills a big share of that. An OLED pixel is done almost at once, so fast motion looks cleaner.
For what OLED does in very dark shades, the OLED near-black uniformity page covers what's normal and what isn't.