Image annotation is the process of labeling objects, regions, or specific features in an image to train machine learning models for computer vision tasks. It is widely used in applications such as object detection, facial recognition, autonomous driving, and medical imaging diagnostics. Common methods include drawing bounding boxes around objects, semantic segmentation (labeling each pixel in an image), polygon annotation for irregular shapes, keypoint annotation for specific points like facial landmarks, and 3D cuboids to understand the dimensions of objects in 3D space.