Amazon have their own music streaming service to rival the likes of Spotify and Napster. Your Unlimited Music Free trial will last 30 days, and is easy to sign up to through your amazon, if you already have one it is a matter of a few clicks.
Being Amazon they have a huge library of popular music. It is pointless listing examples, just know every genre is covered and there is more chance they will have what you are looking for than not to have it.
It is deserving of 4 stars due to its content, however one thing amazon has always struggled with is its user interface has never been great. It is the same with Amazon Video compared to Netflix. Perhaps one day they will have a more dedicated platform for their services outside of the shop.