From time to time, I like to point out a tutorial written by someone else that I personally found to be helpful. This is one of those times.
Colin Smith of Photoshop Café has written may tutorials, as well as produced a large number of videos on both Photoshop and flying drones. They are all quite good. In this video tutorial, he provides a thorough review of Curves in Photoshop. You may find it useful as simply a review, but I bet you will learn something you did not know or have long since forgotten. I know I did.