Basically it's an online radio station tailored to suit you're musical taste. You enter the name of a song or artist that you like. It can't play that particular one straight away (licensing rules) but what it does is find something it knows you will like based on your selection. It will continue to select tracks you'll like, and you can rate each track with a thumbs up or down, enabling it to fine tune your "likes" even more, creating a radio station just for you.
With all the thousands of songs out there it's amazing how it picks the right ones - I find I give the thumbs up to about 9 out of 10. And you can have as many "stations" as you like, reflecting your musical mood of the day.
Give it a try - it's free. You need to register, minimal details required which includes a USA zip-code. I used 300097.