Rotational Velocity and Rotational Frequency

What is the difference between rotational velocity & rotational frequency? Their units seem to be the same, and I've read that one is a 'scalar' and the other is a 'vector,' but how do they differ?

There exists indeed a subtle difference, and are often referred to as angular frequency and angular velocity. Both are $\omega$ and have units $\text{s}^{-1}$.
The angular velocity is actually a pseudovector, and has units $\text{rad s}^{-1}$, but usually the terms are used interchangeably.