Astropad for iPad makes your iPad into a drawing tablet that works with drawing apps on your Mac. Basically, the Astropad app talks to your Mac that is running the drawing app. You see the Mac app on the iPad and can use your Apple Pencil to sketch out what you want on the Mac app.
The app was recently updated to work with the iPad pro and Apple Pencil. You will have to get the Mac version of the app as well because it will talk to the iPad app.
The app will work with other stylus devices. However, working with the Apple Pencil provides some unique advantages because of the way the sensors handle shading and pressure.