ISO image
The ".iso" file extension is the one most commonly used for this type of disc images. The ".img" extension can also be found on some ISO image files, such as in some images from Microsoft DreamSpark; however, IMG files, which also use the ".img" extension, tend to have slightly different contents. The ".udf" file extension is sometimes used to indicate that the file system inside the ISO image is actually UDF and not ISO 9660.