IFO is a type of DVD-Video

DVD-Video

DVD-Video is a consumer video format used to store digital video on DVD discs, and is currently the dominant consumer video format in Asia, North America, Europe, and Australia. Discs using the DVD-Video specification require a DVD drive and a MPEG-2 decoder...

 file that stores information about chapters, subtitles and audio

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 tracks.

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 file types include VOB

VOB

A VOB file is a container format in DVD-Video media. VOB can contain video, audio, subtitle, menu and navigation contents multiplexed together into a stream form. VOB is based on the MPEG program stream format, but with additional limitations and specifications in the private streams...

 and BUP

BUP

A BUP file is a backup of the IFO file on a DVD, which contains the information about the organization of tracks, menus, chapters, subtitles on the disc. BUP files are used in the event that the corresponding IFO file is unreadable, perhaps due to a scratch on the surface of the disc.Other DVD...

.