Standard MP3 players are essentially USB flash drives with an MP3 player built into the BIOS. You can freely exchange any type of file between the two, assuming it will fit (though naturally, you won't be able to play anything but audio tracks). Most use a mini version of good old media player, so basically if you can play it on your PC, it will play on an MP3 player.
The I Pod uses Apple's proprietary DRM technology, which pretty much locks you into using their own store. You can only play music purchased from their store or ripped from your own CD's on the I Pod without hacking the DRM. You can take tunes off, but I don't think the DRM allows you to play them on anything but the I Pod or the PC which purchased the track.
Interestingly enough, Norway recently ruled Apple's fairplay DRM illegal under contractual law, while the EU is investigating it under competition law. Could be fun in the future.