The file formats that are compatible with the player. They include:
MP3--manages to deliver near-CD sound quality in a file that's only about a tenth or twelfth the size of a corresponding uncompressed WAV file.
WMA--tends to outperform some other formats in the area of sound quality, particularly with files encoded at lower bitrates like 64 or 96 Kbps.
AAC--the audio file format used by Apple in their popular iTunes Music Store. Files may appear on your system with the ".M4A" filename extension.