It's basically all aural lessons like intervals, chords, scales, key signatures, pitch and tempo and you can make it as easy or as hard as you
want. The app gives a representation of a piano or a guitar fret board if you want it. I tried the free iPhone version and it was simple and excellent. The full iPad app costs $12.99 but I think it will be well worth it.
Here is a video of it in action:
Guitar Lab - This is a free app that is kind of like a lite version of a series of apps called "50 Guitar Licks You Must Know" like Rock and Blues. It's for guitarists and shows lots of basic and expert guitar riffs that all observe different guitar techniques from different genres with full, patient video tutorials. This app is a catalogue of sorts but has about 8 lessons from each of the genres on offer and the lessons are pretty great as they are videos of a teacher explaining it complete with on screen graphics of what's happening. These other apps are $5.99 each in the app store.
Newzik HD - This is an interesting free app. It could have lots of potential. Watch the video of what this app is all about here.
It should give you an idea of what it does. Only thing is it doesn't have tabs on the app and the such, it uses tabs from various websites so I'm guessing it might be a little hit and miss in terms of good, accurate tabs/notes. The streaming/sharing of the sheet music is very cool though if it works.
Note Perfect! - This is a free iPhone/iPod Touch app I found, it's a simple little game where you get an image of sheet music on the screen (bass, treble...) and you have to choose what note comes up on the scale. It's pretty good for learning notes in a fun way. It can get interesting in the harder difficulty where the notes are several lines above the five normal lines.