The app is free (there’s also a $1.99 version), but you will have to purchase additional categories if you choose to play via the mobile version. The Facebook app appears to be totally free, or at least I haven’t yet encountered restrictions. There are categories for almost every imaginable style of music. Choose your favorite and then listen, and attempt to identify, each of the five songs you will hear a brief clip of. You will only get to hear a couple of seconds of each song, but you do get a multiple choice selection for the answer.
Once you have made your five answers then you can send the challenge to another person and they will get the same category and same five songs to try and identify. If you and your opponent both ID the same number of songs then whoever did it in the least amount of time will be the winner. Your opponent then gets to choose the next category and play the first round.
Get the app from the Google Play Store or from the Facebook App Center.