While it would be unwieldy and almost impossible to discuss every genre of music out there today, several major categories exist in which other subgenres may fall. Perhaps the largest of these music genres is rock and roll, an umbrella genre under which several other types of music can be categorized. Other major music genres include classical, folk, jazz, R&B, blues, metal, and world music.
Rock and roll is characterized by up-tempo beats and guitar-heavy sounds. Within the genre of rock and roll, one will find the music genres of folk rock, alternative, and heavy metal. All of these music genres rely heavily on guitars at various sound levels and distortions; for example, folk rock typically involves the use of acoustic guitars and clean-tone electric guitars at a soft or moderate volume; heavy metal goes in the extreme opposite direction and uses mainly heavily distorted electric guitars. Vocals are often sung loudly or even screamed. Alternative music tends to combine elements of these two genres. Rock and roll was born from the blues genre, as was R&B--short for rhythm and blues.
The glues genre comes from African-American musicians and is characterized by distinct song structures and themes. Blues can be up-tempo or mellow and often focus on some sort of struggle or sorrow; they are often based on spiritual hymns, work songs, chants, or narrative stories. The blues are most often constructed around certain chord progressions called the 12-bar blues. Because of the themes associated with the blues, people will often say they have the blues when they are sad or depressed.
Classical music is one of the major music genres and has been around for centuries. Classical music consists of meticulously composed scores often revolving around pianos and full orchestras. The melodies are often complex and musical scores are oftentimes, but not always, themed. In centuries past, classical music was most popular among the music genres and was prominent in early American, British, and European societies--though again, not exclusively. Today, classical music is commonly heard in movie scores and in local and national symphony halls.
World music is an extremely large genre that encompasses a wide variety of music from around the world. It may include tribal chants, sea shanties, music from various nations and cultures, and anything else that is individual to a certain culture or region. Because this is the largest of the music genres, one might argue it encompasses all music in general; however, in order to be considered world music, a subgenre must demonstrate some sort of regionality or cultural tie.