Alright - short and sweet!
Folk music (also called roots music) is any kind of music learned at home and taught from parents to children for generations. Remember, up until recently that's how people heard music - they played it themselves. They didn't have radios, MTV, CD's, or MP3's. There were no record companies. In America, outside the big towns, they didn't even have many local concerts. Music was whatever instruments you had in your house, and the songs you could play.
Gospel music, African chants, bluegrass, Cajun zydeco, jug bands, and 'old time music' are all folk genres. Even country and the blues started as folk.
The interesting thing about Folk music is how far back it goes. There are musicians in the Appalachian mountains singing songs from old Gaelic that they brought with them from England and Europe hundreds of years ago. Some folk songs are over 1,000 years old! Some folk songs were meant for dancing, but some were ballads, meant as a way of preserving a story for generations.
The most famous folk singer in America was Woodie Guthrie:
Instruments: the guitar, cigarbox guitar, slide guitar, harmonica, bass, and piano. The idea of a small group of musicians, each playing a different instrument has carried over into rock n' roll, and almost every other pop genre.
Cigar Box Guitars
The Blues was a form of folk music that developed in the American south (not South America!) sometime after the Civil War - possibly the 1880's? The first published blues music was in 1908, and the first blues recording was by Bessie Smith in 1920. I couldn't find it on Youtube, but here's another early blues singer:
Singing the blues means singing a sad song (feeling blue means feeling sad - imagine how you feel when you have a bruise).
Blues songs are very simple. You sing a line, repeat it, and then sing a third line that responds to the first. Here's an example by Junior Wells:
“You ain’t nothin’ but a hound dog, babe, come snoopin’ round my door.You ain’t nothin’ but a hound dog, babe, come snoopin’ round my door.
You can wag your tail, but I ain’t gonna feed you no more.”
The Blues almost always use a minor key, and follow a similar pattern of chords, emphasizing syncopation, and "blue notes" - basically, notes with attitude.
This is blues meant for dancing. The most important instrument is the piano, and the most famous singer is Jerry Lee Lewis:
This is just the blues played on amplified instruments. It grew in popularity, starting in Chicago in the 1950's, and is a major precursor to the Hard Rock we know today. Here's a great example by T Bone Walker - you can see where Jimi Hendrix got his inspiration:
Instruments: Trumpets, Trombone, Saxaphone, Clarinet, Piano, Drums, Guitar, Bass
Jazz is hard to define because it keeps evolving. What is jazz? There are different kinds. Jazz grew around the same time as the blues, combining it with ragtime - a kind of classical/pop music that combined African sounds and rhythms with European concepts of melody and harmony. The most important defining aspect of Jazz is improvisation, when a musician invents an original solo on the spot. It's hard to say where ragtime ended and jazz began, but jazz really took off during the years of prohibition in America (1920-1933), when alcohol was illegal and thousands of "speakeasies", illegal bars, sprouted up, each with its own band.
One of the oldest jazz bands
As Jazz became more popular it developed into a dance music called swing, with jazz orchestras called 'big bands'.
Meanwhile, Jazz musicians who wanted to develop improvisation created new kinds, not meant for dancing, like bebop:
and free jazz:
Instruments: Guitar, Violin (fiddle), banjo, harmonica, mandolin, slide guitar, steel guitar
Country music (originally "hillbilly music") comes from old time folk music. One of the earliest country songs, you can hear, sounds just like folk music from before:
Here's America's first nationwide country hit:
Country music was more conservative than other genres. Drums weren't allowed on stage until after 1956. But, country also developed over time into different styles, such as hillbilly boogie, honky tonk, rockabilly, outlaw country, country rock, and country pop which is popular today.
Rhythm & Blues (R&B)
Instruments: piano, organ, drums, bass, saxophone, trumpet, trombone,
R&B has also changed definitions over time. It used to just mean the blues. In the 60's R&B was a combination of blues and gospel, adding back up singers, popularized by Ray Charles.
In the 70's it was synonymous with soul and funk. Nowadays, it mostly refers to Ray Charles and his contemporaries (thanks to a recent biographical film). Contemporary R&B is considered a separate style, with singers like Whitney Houston, Mariah Carey, and Alicia Keys.
Rock n' Roll/Rockabilly
Instruments: Electric Guitar - lead and backup, Bass Guitar, Drums, Snare Drum, Piano
The first Rock n' Roll was very similar to the blues, often playing the same songs, only faster for dancing. While the Blues is mostly in minor keys, Rock n' Roll switched to the major, making songs upbeat and cheerful. Rock music has changed over time, like every other genre, and now Rock is a blanket term for many different styles. The first song to use the term was in 1934:
Here's one of the first songs described as Rock n' Roll, by Sister Rosetta. It starts out a lot like a blues song, and the melody feels like Jazz, with a Jazz orchestra:
Many people feel Rock n Roll first started when Chuck Berry started playing boogie-woogie, switching the piano part for an electric guitar.
Rock n' Roll is famous, not only as entertainment, but as a crosscurrent of racial culture, as music made by black people became popular for whites, who started copying the style. Here's Haley and the Comets