After completing a task you have access to a summary screen that has a lot of useful information.
Under the Overview option you can see how many points and stars you earned; the better you play the more stars and points you receive. You can find out more about how the score is calculated here.
Statistics gives you information about the timing of your play so that you can see where you might need to improve. You can also view the score history here, which shows how many times you have played the song; the higher the bar, the higher the score you received.
You can compare your scores against other players under Leaderboards.
You also have the following additional options on the screen:
Replay the task,