Digital watermarking allows users to embed invisible patterns or data into digital media like images, audio, and video without degrading quality. It works by modifying the media's content data rather than metadata. Watermarking is used to protect copyright ownership of electronic data by making the embedded information inseparable and able to survive various processing without changing the file size. There are different types of watermarks classified by human perception (invisible, visible), robustness (fragile, semi-fragile, robust), and document type (text, image, audio, video). Frequency domain techniques like DCT are often used for video watermarking. The purpose is for copyright protection, fingerprinting, copy protection, and authentication.