Probably a CAD program. You could use a vector graphics app, for example, iDraw is available in the MAS (the devs also have an iOS App). Note: if your drawing is Apple related, iDraw includes shapes libraries & templates for iOS devices, etc., very easy drag & drop. It also has a labeling tool, not something I've used, yet, but it's there.
https://itunes.apple.com/us/app/idraw/id404705039?mt=12