AMOLED is one of two types of OLED display. It is now, by far, the most common type. The AM (active-matrix) type has a transistor next to each pixel, allowing faster response time. This makes AMOLED suitable for displaying video, and is therefore the most common type of OLED display for a main phone display.
The other type is passive-matrix, which is not ideal for video. This type is better suited for small secondary displays.