The Twentieth Century saw incredible advances in technology: antibiotics, flight, rocketry, nuclear physics, television, space travel, computers, the Internet... an exhaustive list could go on and on. Life expectancies and access to goods increased as industrialization improved standards of living worldwide.
Unfortunately, the Twentieth Century also saw more deaths from war, genocide, and oppression than any other period in history, as it became easier and easier for people to tyrannize and kill each other.
Beginning in the early 1900s, it becomes much more difficult to trace the story of music as a continuous narrative, as we largely have until this point. Rather, music, art, and most other human endeavors split into dozens and hundreds of competing "-isms." Expressionism, minimalism, modernism, constructivism, surrealism, post-modernism, just to name a few, can each be studied as separate movements within the artistic world.