"AU" is a file format used for storing sound data on Unix and other systems. The format was developed by Sun Microsystems and is also known as "Sun audio" or "NeXT/Sun audio". AU files typically have a ".au" file extension and can store audio data in several different formats, including uncompressed PCM (pulse-code modulation) and various compressed formats such as µ-law and a-law. AU files are widely used in Unix-based systems and can be played on many media players, including VLC, Windows Media Player, and QuickTime Player. However, due to the popularity of other audio formats like MP3 and WAV, AU files are not as commonly used as they once were.