Software.

iPod can play MP3, AAC/M4A, Protected AAC, AIFF, WAV, Audible audiobook, and Apple Lossless audio file formats. The iPod photo introduced the ability to display JPEG, BMP, GIF, TIFF, and PNG image file formats. Fifth and sixth generation iPod classics, as well as third generation iPod nanos, can additionally play MPEG-4 (H.264/MPEG-4 AVC) and QuickTime video formats, with restrictions on video dimensions, encoding techniques and data-rates. Originally, iPod software only worked with Macs; however, starting with the second generation model, iPod software worked with Windows and Macs. Unlike most other media players, Apple does not support Microsoft's WMA audio format — but a converter for WMA files without Digital Rights Management (DRM) is provided with the Windows version of iTunes. MIDI files also cannot be played, but can be converted to audio files using the "Advanced" menu in iTunes. Alternative open-source audio formats such as Ogg Vorbis and FLAC are not supported without installing custom firmware onto the iPod.
The iPod is associated with one host computer. Each time an iPod connects to its host computer, iTunes can synchronize entire music libraries or music playlists either automatically or manually. Song ratings can be set on the iPod and synchronized later to the iTunes library, and vice versa.
copyrigth http://en.wikipedia.org/wiki/Ipod