On a DVD-Video disc, there are two folders, AUDIO_TS and VIDEO_TS. In the VIDEO_TS folder, there are many files with the extensions .IFO, .BUP and .VOB. The .IFO files are simply information files; they have no video or audio data at all. they hold information on the VOB files, which are the actual MPEG-2 Video, Dolby Digital / DTS / PCM audio files. You don't particularly play an IFO file, but if you were to open an IFO file in certain software it would play the DVD or a certain set of VOB files.
For example, if I were to open WinDVD and drag a VIDEO_TS.IFO onto the interface, it would launch the DVD. If I were to drag something like, VTS_01_0.IFO into the interface, it would play the associated VOB files. IFO files on their own will not play back any video or audio content, they need to be with the other DVD files, in the same folder (in the case WinDVD, it also must be in a VIDEO_TS folder).