OLED (organic light-emitting diode) is an emissive display technology: each pixel produces its own light and can switch off completely, which is why OLED TVs can show true blacks and effectively infinite contrast. QLED (quantum-dot LED), as marketed by brands such as Samsung, is a variation of LCD—an LED backlight shines through a quantum-dot film and other layers to form the image, so it is “transmissive” rather than self-lit.
In practice, OLED usually wins on contrast, black levels, uniformity, and wide viewing angles. QLED/LCD sets often get brighter (helpful in sunny rooms) and are available in a wider range of sizes and price points, especially at very large screen sizes. Both can deliver vivid color and 4K/HDR; picture quality still depends heavily on the specific model.