This is not an app but a new way of thinking about note-taking that relies on the power built into you iPad. "Sketchnoting, in its purest form, is creating a personal visual story as one is listening to a speaker or reading a text." Kathy Schrock.
There is a lot of detail in this iTunes U Course: Digital Sketchnotes for Visualizing Learning
Start with the Overview video below:
Sketchnote Examples:
Sketchnotes from Open Knowledge Foundation Booksprint 3rd September 2013 - From opening talk by Phil Barker and some of my initial ideas about Open Education
Once you've decided to give Sketchnoting a try you might want to watch this awesome video to get an idea how to approach your note taking. It really is awesome. You might also want to try some of the practice activities here.
Use the audio recording functionality in Notability to make it possible to go back and fill in details