As the genre evolved, the music split into a number of different styles, from the speedy, hard-hitting rhythms of be-bop and the laid-back, mellow harmonies of cool jazz to the jittery, atonal forays of free jazz and the earthy grooves of soul jazz. What tied it all together was a foundation in the blues, a reliance on group interplay and unpredictable improvisation. Throughout the years, and in all the different styles, those are the qualities that defined jazz.

Blues
Blues is about tradition and personal expression. At its core, the blues has remained the same since its inception. Most blues feature simple, usually three-chord, progressions and have simple structures that are open to endless improvisations, both lyrical and musical. The blues grew out of African spirituals and worksongs. In the late 1800s, southern African-Americans passed the songs down orally, and they collided with American folk and country from the Appalachians. New hybrids appeared by each region, but all of the recorded blues from the early 1900s are distinguished by simple, rural acoustic guitars and pianos. After World War II, the blues began to fragment, with some musicians holding on to acoustic traditions and others taking it to jazzier territory. However, most bluesmen followed Muddy Waters' lead and played the blues on electric instruments. From that point on, the blues continued to develop in new directions — particularly on electric instruments — or it has been preserved as an acoustic tradition.

Folk
Each country has their own folk music, but the genre usually refers to American and British music that has been passed through the generations by oral tradition. It's simple, acoustic-based music that spins everyday events and common people into mythic status. Many traditional folksongs have no known author, they have simply evolved over the years. Most of the earliest recorded folk music was of this nature, but with Woody Guthrie, topical folk began making its way to record. Still, many artists, including the Weavers and Pete Seeger, chose to mix traditional songs with newer material, either written by the artists themselves or other contemporary musicians. Initially, Bob Dylan functioned in that style, but by his second album, The Freewheelin' Bob Dylan, he began relying entirely on original material, thereby ushering the modern era of folk, where most performers sang their own (usually personally and introspective) material, and only occasionally throwing in covers.

Rap
To the untrained ear, all rap and hip-hop may sound the same, but there's a number of different levels in even the simplest rap song.