Free jazz is a style of music developed in the 1950s and 1960s and pioneered by artists such as Ornette Coleman, Cecil Taylor, John Coltrane, Eric Dolphy, and Albert Ayler. It uses jazz idioms but considerably less compositionalal material than in most earlier styles -- improvisation is essential to it, and whereas in earlier styles of jazz the improvised solos were always built according to a template provided by composed material (chord changes and melodies), in free jazz the performers often range much more widely. Like all forms of jazz, free jazz still has its practitioners today.
Typically this kind of music is played by small groups of musicians. In popular perception, free jazz is loud, aggressive, dissonant and in general full of sound and fury. Many critics, particularly at the music's inception, suspected that the abandonment of familiar elements of jazz pointed to a lack of technique on the part of the musicians. Today such views are more marginal, and the music has built up a tradition and a body of accompanying critical writing. It remains less popular than some other forms of jazz.
Beyond this, free jazz is most easily characterised in contrast with what we refer to here as "other forms of jazz", an umbrella which covers ragtime, dixieland, swing, bebop, cool jazz, jazz fusion and other styles, as in the following paragraphs.
"Other forms of jazz" use strongly-pulsed metrical rhythms, usually in 4/4 or (less often) 3/4. Free jazz normally retains a general pulsation but without regular metre, and often with frequent accelerando and ritardando, giving an impression of the rhythm moving in waves. Often players in an ensemble adopt different tempi. Despite all of this, it is still very often possible to tap one's foot to a free jazz performance; rhythm is more freely variable but has not disappeared entirely.
Other forms used harmonic structures (usually cycles of diatonic chordss). Improvisors played solos using notes based on the notes in the chords. Free jazz almost by definition dispenses with such structures, but also by definition (it is, after all, "jazz" as much as it is "free") it retains much of the language of earlier jazz playing. It is therefore very common to hear diatonic, altered dominant and blues phrases in this music. It is also fairly common for a drone or single chord to underpin a performance (see modal jazz), but the absence of such rudimentary devices is also common.
Finally, other forms use composed melodies as the basis for group performance and improvisation. Free jazz practitioners sometimes use such material, and sometimes do not. In some music which is called "free jazz", other compositional structures are employed, some of them very detailed and complex; the music of Anthony Braxton furnishes many examples. It would perhaps be best to call this modern or avant garde jazz, reserving the term "free jazz" for music with few or no pre-composed elements.