YOLO (You Only Look Once) is a method / way to do object detection. It is the algorithm /strategy behind how the code is going to detect objects in the image/video.YOLO is a great example of a single stage detector.
First introduced in 2015 by Redmon et al., their paper, You Only Look Once: Unified, Real-Time Object Detection, details an object detector capable of super real-time object detection, obtaining 45 FPS on a GPU.
YOLO has gone through a number of different iterations, including YOLO9000: Better, Faster, Stronger (i.e., YOLOv2), capable of detecting over 9,000 object detectors.