I have a hard time understanding how to find the domain of functions.

Here are two samples:

Find the domain of each function below.

(a) u(h) = sqrt{h^2 - h - 2}

(b) g(k) = k/(sqrt{k - 4})

Must I rationalize the denominator for question (b) as a first step?

What exactly is a DOMAIN anyway?

How is it different from the range?