If you agonize over music for parties, or if your friends take control of your music when you have a party then you need No DJ.
Create a facebook event (using your web browser or using No DJ) and invite your friends. Thats it, no more planning music.
No DJ will go off and find the music that your friends like, and then create a suggested playlist for you. You can customise the playlist, or leave it as is, safe in the knowledge that people you have invited have indicated they like your music.
Once the suggested playlist has been created, No DJ will play any items in the playlist that you already own, and make suggestions for songs that you should own based on what your friends like.