The 90s didn't invent new styles of music, but rather combined music styles that had been popular in the 70s and 80s. For example, punk and heavy metal combined to make punk rock, and R&B combined with hip-hop to make West Coast style rap. http://channel.nationalgeographic.com/the-90s-the-last-great-decade/articles/the-90s-music/
Stars and groups that we know and love today were also popular during this time, like Mariah Carey, Mary J. Blige, U2, and Nirvana.
Many concept albums that were created during the 1990s hit many of the same themes that are typically associated with this decade. Concept albums during this time period were filled with angst, futuristic thoughts, irony, and other heavier emotions and ideas that step outside the "pop" norm.
Listen to this mashup of popular 90s music!