MP3 files contain compressed audio data. They use MP3 compression, which is one of the most common audio compression algorithms. The compression rate can vary between MP3s, which may affect the file's size and audio quality. Higher compression will result in smaller files with lower fidelity, while lower compression will produce better-sounding files with larger file sizes.

MP3s were the first major compressed audio format, but have become less popular since the introduction of the AAC codec, which provides better audio quality at similar compression levels. (These files use the .AAC extension as well as several other file extensions.) However, since MP3 files are still ubiquitous, they can be played by nearly all modern audio applications.