AU is an audio file format developed by Sun Microsystems for storing sound data. It is commonly used in early internet applications and on Unix-based systems. AU files are simple and contain compressed audio data, making them useful for storing and transmitting sound on limited bandwidth networks.
MP3 is an audio format that can compress and encode an audio file. It uses the lossy compression algorithm to contain audio data. The lossy compression helps to reduce the data significantly without losing audio quality. MP3s are created using different bit rates maintaining the original quality.