m4v is used in Apple iTunes and iPod. It can be renamed to MP4 file extension. MP4 is a container format. As any other container format like AVI (.avi), RealMedia (.rm, .rmvb), QuickTime (.mov), MPEG (.mpg, .mpeg), Matroska (.mkv, .mka), OGM (.ogm) it allows you to combine different multimedia streams (mostly audio and video) into one single file.