Swing jazz was popular during the 1920s and '30s and was performed by larger "Big Band" ensembles, though some small groups performed this music as well. The leading musicians of this time were Louis Armstrong, Duke Ellington and Count Basie.
Bebop jazz became popular in the mid-1940s in New York and was characterized by its fast tempos, musical virtuosity and extended improvisations. Charlie Parker and Dizzy Gillespie were the leading bebop jazz musicians.
Hard bop developed in the mid-1950s and was in style until the mid-1960s. Hard bop used complex chord changes and sophisticated harmonic techniques and was characterized by players such as Miles Davis, Wes Montgomery and John Coltrane.
Free jazz grew out of Hard Bop in the mid-1960's and used a highly creative, stream-of-consciousness style of improvisation that often involved using non-musical elements in the music. Albert Ayler and John Coltrane were influential free-jazz musicians.
Fusion jazz grew out of the mixing of jazz and rock music styles and was pioneered by Miles Davis on his album "Bitches Brew" in the late 1960s. Fusion used rock rhythms and jazz chord progressions to produce a high-energy, psychedelic form of jazz.
Jazz since the late 1970s is labeled as "modern jazz." Modern jazz takes influences from all of the previous genres of jazz, and some of its biggest proponents include Pat Metheny, Joshua Redman and Brad Mehldau.