What is Jazz? There are several, if not many answers, but in a nutshell, it encompasses several styles:
1. Ragtime - in the teens and 1920s, ragtime began to evolve. In New Orleans, Dixieland jazz added larger orchestration and the basic elements of swing.
2. From the late 1920s through the 1950s, big band jazz was the predominant form - from Paul Whiteman and his Orchestra to Glen MIller and everything in between. This is when swing really began to develop.
3. In the late 1940s, Miles Davis et al made "cool jazz" popular (Birth of the Cool) - smaller bands became more common.
4. By the mid-1950s, a response to cool jazz - "Hard Bop" - came into being. More aggressive than cool jazz. Around this time, the "cool" side was generally associated with the West coast of the US and the harder side was associated with the East Coast - more to the point, LA vs NY.
5. By the 1960s, jazz became even more adventurous with Free Jazz (e.g., Ornette Coleman) and later, jazz fusion (e.g., Miles - Bitches Brew).
6. The 1970s brough a growth of fusion and a greater mixing of rock and roll and jazz (e.g., Zappa).
7. By the late 1970s and into the 1980s (go figure), smooth jazz began to appear.
8. These days, jazz is a varied mix of world beats, including Latin and African, straight-ahead swing and many other improvisational styles.
So, that's what jazz is to me, but a simpler answer would be: listen to Mingus.
Here:
Charles Mingus - Groove!