how do they know?

So the webpage I was just looking at had little advertisements imbedded in the page. Normal, right? Yeah... but I actually took the time to read those ads today, and they were creepily catered to me. Ok, maybe not the one about getting a credit card, but the one advertising the "Magic Dance" ringtone?! Man! How do those little spies know I am a closet Labyrinth fan? That is just sick. I mean how many people would even know that "Magic Dance" is a David Bowie song in Labyrinth?