>>16727040
and that's it.
it's shit for intuition, but derives the rotation formula using the bare minimum of concepts.
this gives you enough to code up your own quaternion rotation transformation library.
quats are good for rotations because they don't suffer from gimbal lock when tracing out paths in rotation space, and they have useful interpolation methods with useful properties like spherical interpolation

as for graphics, this is pretty much all you will ever need to know about quaternions.
you use them to represent rotations, but they all get mulched into real matrices at the end of the day when the time to apply the transformation comes.