Question - what does free jazz mean? Play a tune (precomposed) then improvise without regard to any form? Or are there constraints on the improvisation too? I mean, I have jammed freely with various folks over the years, sometimes it came out sounding like Ornette and sometimes it came out sounding like the blues.
There are a lot of great suggestions in this thread. A couple topics I wanted to talk about though. This is a long post yet way too short at the same time...
"free jazz" is certainly a vague term and not one used or really accepted by many of the main practitioners. I was speaking recently with William Parker and Hamid Drake and they don't like that term. They don't even like the term jazz which originates from a slang term describing African Americans having sex. To be honest, I don't remember what terms they do use for their music, I think they just avoid genre specifications.
Be that as it may, we can speak of "free jazz" as a sort of genre based on music that's happened since the late 50s. Of course Lennie Tristano, Lee Konitz and Warne Marsh experimented with fully improvised music in 1949.
The freedom in free jazz can reflect many things. The most shocking initial difference between free jazz, and why Ornette angered so many folks when he showed up on the scene, was the lack of precomposed chords one had to follow while soling. Some of his compositions did have chords for the melody, but after the melody everyone was free to move forward as they saw fit. The thing people don't realize here is just because no chords were written doesn't mean the music was atonal or the people played whatever they wanted. Much of Ornette's playing is diatonic and heavily influenced by the blues. You hear a strong connection harmonically between Charlie Haden and Ornette, or Don Cherry. This, in my mind, is kind of one category of free jazz. Improvising very much in the jazz idiom. Often the music swings and there is a pretty clear tempo, though it may switch suddenly. However, when it does, the band follows. This music is all about listening and interaction. You really need to hear harmonically what everyone is doing to follow, contrast, whatever you decide. Rhythmically it's mostly a similar approach where you have freedom to go anywhere, but it's about going there organically as a group
Then you had someone like Albert Ayler who was stretching beyond harmonic implications of notes and really moving towards playing with pure sound. Anthony Braxton speaks of saxophone pre and post Ayler as this really signified that split from playing notes to pure sound. Timbre, dynamics. There's still an incredible sense of interplay but it's more gestural than finding harmonic ideas together as a group. Sunny Murray is also pivotal in that he completely freed up the drum set from it's role as a time keeper. Here, as well as his work with Cecil Taylor, is where you start to hear truly non metric music.
Cecil Taylor and John Coltrane also largely made music based on specific harmonic ideas. There is a great transcription in the Lewis Porter book of, I forget, Venus? From Interstellar Space where he shows the specific harmonic ideas being clearly outlined by Coltrane throughout the piece. There is nothing random, or even atonal here, he's bringing all of his harmonic knowledge from his life as a musician to improvise a piece. There approach to time is an interesting one. Rashied has a wonderful way of providing pulse without a clear metric time feel. I'm told he claimed he played time. I was just speaking with a fantastic British drummer, Andrew Lisle, about Rashied. I should talk to him more, but basically he was saying Rashied had a wonderful way of playing with so many layers and time implications that basically anything Coltrane could play would fit and he'd be able to connect.
DDGsax mentioned Braxton's compositions and suggested they were minimal and abstract. Graphic scores and the like. While that is true in some cases, Braxton employed just about every compositional tool imaginable, many novel, to find new ways to explore music. He has a LOT of fully notated compositions. I'm talking hours and hours and hours of thoroughly notated, almost unplayable, difficult music. Some of it sounds like aleatorical music, some stems directly from jazz, ie the language of Charlie Parker or Lennie Tristano. With our Amsterdam based collective Doek we did a collabopration with Braxton for his 80th birthday and I was lucky enough to play a concert of his music along with the other Doek members and 10 members of his collective Tri-Centric. I was relieved to be playing baritone as the alto and tenor parts were extremely challenging. Not quite Fernyhough, but a good few steps more difficult than the Omnibook. The altos and tenors included Michael Moore and Ingrid Laubrook and they were struggling... However, Braxton didn't mind the result at all. As a matter of fact, on this fully notated piece we were also encouraged to play in any transposition we chose. Of course, I doubt anyone with those super fast, tristano through a meat grinder lines, attempted to play it in another key... But that was encouraged. Another piece was fully notated but done so in a way it was basically impossible to keep track of what was your line and what was someone elses. In rehearsal we were all a bit anxious, trying to figure out how to play it "correctly". We'd start a section, muddle our way through to the pause. Sit there in confusion, ashamed we couldn't figure it out just to hear Anthony say "oh wonderful, wonderful!! just beautiful! Ok next section".
The point of all this is "free jazz" can mean anything. It's just about finding ways of breaking out of the rules and creating new music. Creating the music you want to make. Some musicians work very hard to avoid any harmonic implications, some musicians seemingly try to avoid having their instrument sound like their instrument, while others are happy to groove, go into a blues, whatever. Playing with William Parker and Hamid Drake is a challenge because they are some of the very few people I know in the music that are totally free to go anywhere. They can play as abstractly anyone then suddenly slip into a reggae. It's all music and the boundaries are imaginary.