|Pet owners are usually well aware of how abundant fur can be|
As we all know, hair is something that only mammals have. It serves some of the same purposes as scales and feathers, but is definitely unique. The main function of hair is insulation and thus temperature regulation, which is vital for endotherms to maintain homeostasis. Interestingly enough, scientists haven't been able to pinpoint when it evolved, including whether endothermy or hair were first on the scene.
|Anatomy of a hair follicle|
Now, let's move on to types of hair. There are more than you might have originally thought. There are two basic types of hair with more specific types fitting into one of the two. Angora hair is hair that never stops growing and which may or may not shed. This would be what humans have on their heads, what horses have in their mane, and what certain breeds of dog have all over their bodies. In contrast, there's definitive hair, which will only grow so long and which shed and re-grow regularly. This would be what's on the body of a cat and most dogs. Within angora and definitive hair, there are a number of other relevant hair categories, and this is where it starts to get complicated.
|Vibrissae on rats, along with pelage|
|Horses have numerous types of hair: vibrissae, pelage (both guard hair and undefur), and bristles|
Source is Mammalogy: Adaptation, Diversity, Ecology. Images are from Wikimedia Commons and are under Creative Commons licenses or are copyright free: one, two, three, four, five, six.