Jazz is a style of music which emerged in America around the turn of the nineteenth/twentieth century. There are many styles of jazz, but broadly speaking jazz is characterised by improvisations, syncopations (offbeat accents) and swung rhythms.
Improvised playing is not based on a written score but on the mood of the moment.
Black music came over to America with the African slaves. African music combined with the music of the white European settlers to produce new styles of music including blues and ragtime.
Towards the end of the nineteenth century marching bands were popular in America. Black musicians began to jazz up the marches, adding syncopated rhythms, 'bending' notes and improvising on the melodies.
Trad (traditional) jazz emerged in the early twentieth century in Storyville, the red-light district of New Orleans.