What is semantic segmentation vs object detection?

Object detection identifies the location (bounding boxes) and class of each object in an image. Models: YOLO (real-time), Faster R-CNN, SSD. Semantic segmentation assigns a class label to every pixel in the image but does not distinguish between instances of the same class (all cars are one "car" region). Instance segmentation (e.g., Mask R-CNN) combines both — it detects individual objects AND segments each at the pixel level. Panoptic segmentation is the most comprehensive, covering both "things" (countable objects) and "stuff" (background regions like sky, road).