There are variables assigned to describe pitch, timbre, and more esoteric descriptors like mood and 'danceability.' It's even possible to derive realtime volume information, all without processing the audio stream directly. Each song within Spotify's library has been fully analyzed: broken up into individual beats, segments, tatums, bars, and sections.
Having been acquired by Spotify, their analysis resources are available via the Spotify API. The Echo Nest represents the comprehensive algorithmic analysis of music. A WebGL Spotify visualizer made with Vue, D3, and Three.js.