ART is a new runtime aiming to replace the dalvik, which is responsible for running apps on Android. ART is supposedly going to be much faster, smoother and just better. The thing is, it's not official yet. It's just "beta" and cannot be used officialy now. However, you can enable this feature in developer options in settings.
The result is satysfying, apps are launching much faster, loading takes less time and it's just generally better.
Try for yourself, you might like it!