Rock Rock & Roll is often used as a generic term, but its sound is rarely predictable. From the outset, when the early rockers merged country and blues, rock has been defined by its energy, rebellion and catchy hooks, but as the genre aged, it began to shed those very characteristics, placing equal emphasis on craftmanship and pushing the boundaries of the music. As a result, everything from Chuck Berry's pounding, three-chord rockers and the sweet harmonies of the Beatles to the soulful pleas of Otis Redding and the jarring, atonal white noise of Sonic Youth has been categorized as "rock." That's accurate -- rock & roll had a specific sound and image for only a handful of years. For most of its life, rock has been fragmented, spinning off new styles and variations every few years, from Brill Building Pop and heavy metal to dance-pop and grunge. And that's only natural for a genre that began its life as a fusion of styles.
COUNTRY Country music is about tradition, yet its simple form lends itself to endless variations on similar themes. Like blues -- the two genres often shared themes, melodies and songs -- country is a simple music at its core. Most of its songs are built around three chords and a plain melody, but these forms are so basic, they allow for any different styles, from the gritty sounds of honkytonk to the jazzy improvisations of Western Swing. Country music grew out of American Southern folk music, both Appalachian and blues, and old-time country was simple and folky, with just guitars and fiddles. As the genre progressed, old time music evolved into the rhythmic guitar-and-fiddle driven traditional country that became the foundation of modern country music, from honky tonk and Western Swing to the pop-oriented Countrypolitan and rock-inflected Bakersfield Sound.
JAZZ Jazz has been called America's classical music, and for good reason. Along with the blues, its forefather, it is one of the first truly indigenous musics to develop in America, yet its unpredictable, risky ventures into improvisation gave it critical cache with scholars that the blues lacked. At the outset, jazz was dance music, performed by swinging big bands. Soon, the dance elements faded into the background and improvisation became the key element of the music. 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, atonalforays of free jazz and the earthygrooves 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, ruralacoustic 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.
NEWAGE Born from an aesthetic that aims to induce a sense of inner calm, new age music emerged from the meditational and holistic fields. Generally, these are harmonious and nonthreatening albums that are allied with new age philosophies encouraging spiritual transcendence and physical healing. Some of these albums are artistically satisfying as well as therapeutic. Lesser musicians, however, often make ridiculous claims in the liner notes as to their ability to catapult listeners into advanced spiritual states through specially designed sonicvibrations and "immaculately conceived" musical ideas. ~ Linda Kohanov
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. At its core, hip-hop is a post-modern musical genre that deconstructs familiar sounds and songs, rebuilding them as entirely new, unpredictable songs. Early rap records, commonly called "old school," were made by DJs scratching records and playing drum loops, with MCs rapping over the resulting rhythms. As the genre progressed, hard-rock guitars and hard-hitting beats were introduced by Run-D.M.C., the first hardcore rap group, and the scratching techniques were replaced by sampling. With their dense collages of samples, beats and white noise, Public Enemy took sampling to the extreme, and they helped introduce a social and political conscience to hip-hop. That faded in the '90s, as gangsta rap -- originally introduced by NWA, who used Public Enemy's sound as a template -- became the dominant form. By the '90s, gangsta rap, which originally was in direct opposition to such pop-oriented rappers as MC Hammer, had become smoothed over and stylish, and consequently was more popular than ever, as evidenced by the success of pop-gangsta Puff Daddy.
REGGAE Reggae is a music unique to Jamiaca, but it ironically has its roots in New Orleans R&B. Reggae's direct forefather is ska, an uptempo, rhythmic variation based on the New Orleans R&B Jamaican musicians heard broadcast from the US on their transistor radios. Relying on skittering guitar and syncopated rhythms, ska was their interpretation of R&B and it was quite popular in the early '60s. However, during one very hot summer, it was too hot to either play or dance to ska, so the beat was slowed down and reggae was born. Since then, reggae has proven to be as versatile as the blues, as it lends itself to a number of interpretations, from the melodic rock steady of Alton Ellis and the rock and folk-influenced songwriting of Bob Marley to the trippy, near-psychedelic soundscapes of dub artists like Lee "Scratch" Perry. It has crossed into the mainstream through the bright, bouncy "reggae sunsplash" festivals and pop-oriented bands like UB40, but more adventurous reggae artists, such as Marley and Perry, have influenced countless reggae, folk, rock and dance artists. Their contributions resonate throughout popular music.
This article is a collage of brief introductions taken from All Music Guide.