Types of human hair
There are many types of human hair and it differs in color, texture, length, diameter and even straightness. A persons hair changes as they age, sometimes changing color and even going from curly to straight or vice-versa.
Adults have two types of scalp hair. Vellus hair and Terminal.
Vellus hairs are short fine hairs that normally are called peach fuzz. They are typically a centimeter or two long and contain no pigment.
Terminal hairs are long pigmented hairs that grow on the head and in men and some women on the face, body, arms and legs. The process of male pattern baldness will cause Terminal hairs to become Vellus hairs over many growth cycles. This can be reversed to some extent through treatments if the treatment is started early enough.
During childhood the scalp consists of both types of hair Vellus and Terminal and as the person ages and enters purberty most of the vellus hair will morph into terminal hairs. Terminal hairs also begin to appear in the armpits, groins and legs, and also (in males) on the chin, chest and forearms at puberty due to the effect of male and female sex hormones.
As humans age the hair will begin to thin as a normal aging process and by the time they reach 70 to 80 many have pronounced hair thinning over their entire scalp. This is a normal process and not associated with male pattern baldness but with the normal aging process.
There are also differences in the straightness of hair.
An example of curly hair
An example of straight hair
In this example, the cross section would be round judging from the extreme straightness. I've found that many people would prefer to have what they do not, so most people with curley hair would prefer straight hair and vice-versa. And of course most balding men would prefer to have hair period.