Quaternions are non-commutative (which means that factorial order is important) complex numbers with one real component (w) and 3 imaginary parts, useful in spatial dynamics and physics for calculating orientation.
Angle-axis, on the other hand is a one-dimensional rotation around an arbitrary three dimensional axis.
While the number of components needed to represent an orientation are the same in both cases, they are not immediately interchangeable.
Angle-axis, on the other hand is a one-dimensional rotation around an arbitrary three dimensional axis.
While the number of components needed to represent an orientation are the same in both cases, they are not immediately interchangeable.