Jazz (also known as "jass" in it's earlier years) is a genre of music that originated in the United States in the early 20th century from European music and the African musical traditions of American slaves. Among the hallmarks of jazz are the use of uneven, or "swung," eighth notes, heavy rhythmic syncopation, altered, or "blue," notes, and a focus on improvisation.
The music of 19th century slaves in the American South was rooted in the musical traditions of their native Africa and was characterized by call-and-response structure, single-line melodies, and use of pentatonic scales. The combination of this rich musical tradition with the instruments and harmony of European music laid the foundation for what would eventually develop into jazz. While jazz and blues music are closely related, the terms are not at all synonymous. Blues developed more-or-less directly from the work songs, field hollers, and spirituals sung by slaves and shows much less of the European influence that is characteristic of jazz. However, the blues "feel" and the 12-bar blues song structure have always been important parts of jazz music.
Since its early years, jazz has influenced and been influenced by other forms of popular music and has evolved and branched out into numerous sub-genres. Below, in more-or-less chronological order, is a list of some of the major developments in the genre with some brief descriptions:
Dixieland jazz, or New Orleans jazz, was one of the earliest forms of jazz to achieve widespread popularity, aided by advances in the production, distribution, and affordability of sound recordings and equipment. Dixieland developed in New Orleans early in the 20th century, and by the end of the First World War and the onset of the "jazz age," it had spread to much of the rest of the nation. It is characterized by simple harmonies, extensive use of the twelve-bar blues form, a "hot" swing beat, and collective group improvisation.
Swing music rose to prominence in the 1930s and dominated popular music for many years. As its name suggests, the "swing" beat of dixieland jazz was retained. The focus on collective improvisation, however, was not. Many swing groups were "big bands" of 12-20 musicians or more and the music was highly orchestrated, leaving much less freedom for improvisation. One exception was Kansas City swing, in which the groups were smaller and more attention was given to individual improvisation.
Bebop arose towards directly out of KC swing at the end of the 1940s and quickly exploded in popularity. Unlike many earlier forms of jazz which usually featured vocalists prominently, bebop was primarily an instrumental artform. It is characterized by fast tempos, sophisticated harmonies, and virtuosic improvised solos. Due to it's poularity and profound influence on future musicians, the development of bebop is often seen as the beginning of modern jazz.
Cool jazz or west coast jazz developed in the 1950s as a reaction to the frantic pace and complexity of bebop. Cool jazz features slower tempos, simpler and less frequent chord changes, and less complex improvised solos.
Free jazz was another reaction to the complexity of bebop. In free jazz, most or all of the traditional "rules" of harmony, melody, and rhythm were abandoned in an attempt to grant the musicians total freedom. Free jazz also marked a return to the focus on collective improvisation that was a prominent feature of early jazz.
Fusion arose in the 1960s as jazz musicians began to incorporate outside influences into their art. Rock fusion, latin fusion, and bossa nova, a fusion of jazz instrumentation and harmony with traditional Brazilian rhythyms, were widely experimented with. Fusion reached it's peak in the 1970s, eventually dying down and giving rise to new jazz-influenced genres such as funk, smooth jazz, hip-hop, and acid jazz.
The above are just a few of the more prominent and historically important sub-genres of jazz music. There are many more that could have been mentioned and, as the artform continues to develop and evolve, there are sure to be many more in the future.