Spotify CEO (Daniel Ek) explain in brief how Spotify’s Discovery Weekly playlists work.

In short it’s based on three simple parts: your playlists, your taste profile and our huge library. Spotify generates hundreds of terabytes of data every day that we analyze in order to understand that you might actually be interested in a song that we just added by an unknown band that has characteristics that you and 100 other people in the world like. When you find that needle in the haystack – magic happens. And for us this is just the beginning.

If you’d like to learn more, this article is a pretty good primer. http://qz.com/571007/the-magic-that-makes-spotifys-discover-weekly-playlists-so-damn-good/