Mohair is not wool, as such. Wool, strictly speaking, is a seasonally-growing undercoat from sheep, although the term is often used more widely to include fluffy undercoats from many other animals, including yak and buffalo.
Mohair is a hair fibre from the Angora goat. It has a silk-like sheen, dyes well, and does not felt. It is also very sturdy compared to similar-quality sheep’s wool, and for that reason is often used to make outerwear, and, historically, soldiers’ uniforms.