IFO is a DVD InFOrmation file that stores information about Chapters, Subtitles and Audio Tracks.
IFO information is a Video Title Set (VTS).
An IFO file is one of three types of files that are used on DVDs. It contains important navigational information, such as where a video chapter begins, and where audio and subtitle streams exist within the movie (VOB) file. IFO files are not encrypted.
Other DVD files include VOB and BUP files.