Hair and fur are two words for the exact same thing: thin fibres made from the protein keratin. These fibres grow out of organs called follicles, which you’ll find in the skin of nearly all mammals, from mice to elephants. The only difference between hair and fur is the kind of mammal you’re talking about. Fur is reserved for non-human animals, while hair can mean fibres on either animals or humans.
Of course, a human head of hair and an animal’s fur coat do have some key differences. A typical furry animal, like a cat, sports a dense coat of very fine individual fibres. Human heads have a less dense coat of thicker individual hairs. Additionally, the hair on our heads grows to greater lengths thanks to a unique activity cycle.
The hair on your arms or legs, akin to animal fur, is in the growth stage (called anagen) for 19-26 weeks before reaching an intermediate phase (catagen) followed by a shedding phase (telogen). In contrast, the hair on your head might be in the anagen phase for two to six years. The follicles and fibres themselves are the same, but our genes make them follow different cycles.
Answered by Tom Harris