Spotify - how does it work?

[rupert the dog]

I've been on Spotify since it started,and I think it's fantastic that I can listen to almost any music that I want to, for free, just for the sake of a few ads now and then. But I don't understand how it works. Thousands (millions?) of people all listening to their choice of music at the same time?? Black magic voodoo, someone tell me how they do it.