OLED panels are mostly found in some high-end TVs, several Android smartphones, and most recently in the Apple iPhone X.
In general, OLED panels are better than LCD (liquid crystal display) panels in almost every way. They offer incredibly rich color reproduction compared to LCD panels you'd find in older Android phones and the iPhone 8, as well as better contrast between bright and dark areas.
OLED panels can also reproduce a perfect black color because the pixels displaying the color black will actually turn off entirely. When an LCD panel displays the color black, it tends to appear more gray because the entire LCD panel is constantly backlit, even when there's the color black being displayed. The liquid crystals in front of an LCD panel's backlight can't block out the backlight entirely, and thus the color black can rarely ever be properly reproduced.
By allowing pixels to turn off entirely for the color black, OLED panels can improve the battery life of mobile devices, too, especially when viewing darker content.