MDF (Media Disc Image File) and MDS (Media Descriptor File) are a pair of file formats used for creating and managing disc images, primarily for optical media like CDs and DVDs. The MDF file contains the actual raw data of the disc, essentially a complete sector-by-sector copy. The MDS file, on the other hand, is a small descriptor file that stores metadata about the disc image, such as track layout, session information, and other properties. It acts as a table of contents for the MDF file, allowing software to properly interpret and mount the image. When mounting or burning an MDF/MDS image, both files are typically required. The MDS file tells the software how to handle the data contained within the MDF file. This format was popularized by Alcohol 120% and is often used for backing up game discs, software installations, and other optical media. While not as universally supported as ISO images, MDF/MDS offers features like support for multiple sessions and data tracks, making it suitable for complex disc structures. It's important to keep both files together for the image to be usable.