MPEG started as the name of a group of engineers who write rules for packing moving pictures and sound into computer files (Moving Picture Experts Group). People then used MPEG as the name of the files themselves — a clip of video-plus-sound on a computer or on the internet. You may see .mpeg or .mpg on the end of a filename. MP3 music files come from the same family of rules, just the sound layer. Newer video you watch online is often a grandchild of this work, even if nobody says “MPEG” out loud any more; MP4 is related. You say it “em-peg.” It is a tech word from the 1990s that still turns up on old files and in manuals.