As with Smule's other offerings, you can buy various songs with in-app purchase, and there are a few standards included for free. You can sing along solo, or open up your session for others to listen to and join. That's fun, but the social aspect of Sing! sets it apart.
You can browse other people's performances on a global map, listen to karaoke sessions from around the world and comment on your favorites. You can even add your own track to someone else's recording (with songs you haven't yet purchased). For example, you can sing backup on someone else's tune, or just the verses of a rap song, leaving the chorus open for someone else.
It's a fun time, and thanks to Smule's hard work on music sharing technology, it all works amazingly smoothly. Uploading and downloading songs and performances all happens in a snap. Smule has been at this for a few years now, and they've mastered this kind of sharing. Even if you're not interested in performing any karaoke, it's fun to hear what others are sharing.