Also see XSPF v1 Notes and Errata
What is XSPF?
XSPF is an XML playlist format. It is different from other formats in a few ways. One, the only open/nonproprietary format is SMIL, but SMIL is really not a playlist format, it's an idea for a type of application, and that application is not the same kind of thing as an MP3 player. Two, the only other playlist format that the drafters did a really careful job with is SMIL, all the others, like M3U, are so half assed that they have a lot of unfixable bugs. Three, XSPF is very internet friendly, so playlists can go from one machine to another.
What's the file extension of XSPF files?
.xspf and only that.
Are nested playlists possible?