Since Pandora started back in 2000, they’ve been working on the Music Genome Project. It's the most comprehensive analysis of music ever undertaken. They put together a team of music-analysts and had them listen to music, one song at a time, studying and collecting literally hundreds of musical details on every track - melody, harmony, instrumentation, rhythm, vocals, and lyrics. Those details were then mapped to millions of songs on a catalog and continually updated as new music comes out.