If you want to use your iPad (and iPad Pro) to access your Mac display to draw on, get Astropad. You need to have both the Mac component (free) and the iPad app ($20) in order to get it work. With this you can have a true Wacom Cintiq experience where your tablet is your Mac screen. Here is a video with a small demo on the iPad Pro and another one comparing the iPad Pro to a Cintiq.
/via Daring Fireball