An annotation tool is a specialized software platform designed to facilitate the efficient and accurate labeling of data for machine learning projects. These tools provide a user-friendly interface that allows human annotators to interact with raw data, applying labels through features like bounding boxes, polygons, keypoints, semantic segmentation masks, and text classification tags. The sophistication of an annotation tool can range from simple, open-source applications for basic tasks to enterprise-grade platforms that support complex workflows, real-time collaboration, and advanced automation.