To go along with the other music threads, I'll add one on jazz.
I know very little about jazz, even if I did watch whatever I could catch of the PBS series on jazz.
...
What else makes it jazz?
I didn't see Andie's jazz thread before starting this one. Sorry!
I think Andie's thread was about
singing jazz while playing uke. Your question seems more general, about the genre of music itself, so I'll post a few thoughts here.
That Ken Burns documentary is a great place to start - it's on Amazon and iTunes and I highly recommend a re-watch if you are so inclined. There were also CDs released that went along with it, including an overview with various artists. One of my favorite takeaways from it were two quotes from Duke Ellington, both of which I'm not going to do justice to, but I'll try: first, he was known for saying that there are only two kinds of music:
the good kind, and the other kind. The other was someone reminiscing that the highest compliment Ellington could pay a musician was that he or she was
uncategorizable.
So with that in mind - I find it hard to define exactly what makes one genre of music different than another (the folk music thread is a great example of this - lots of blurred lines!). But I'd say for jazz, one of the key elements would be syncopation, rather than playing it straight. Within the categorizable genre of "jazz" itself would be some very different styles - ragtime, Dixieland, bebop, vocal jazz, hot jazz, cool jazz, West Coast jazz, swing, bossa nova... and more.
A partial list of my favorite artists that would conventionally be categorized as "jazz" would include Ellington, Billie Holiday, Ella Fitzgerald, Louis Armstrong, Sarah Vaughan, Dinah Washington, Benny Goodman, Chet Baker... the list could go on. But to illustrate the "uncategorizable" side, all of these artists crossed over into other genres, often unexpectedly. Sarah Vaughan covered the Beatles on one album. Louis Armstrong did Disney movie songs on another.