People around the world have different kinds of hair, but in the broadest category, hair is either straight or curly. It can depend on a lot of things, and is even different based on population. People from countries like Japan, China and Korea tend to have mostly straight hair, while Africans have curly hair and Europeans are mostly a mixed bag.
The reasons for this are numerous and varied, but in a basic sense, if your parents have curly hair, then you're likely to have curly hair, and likewise for those with straight-haired parents. If one parent has straight and the other has curly hair, the child will likely have wavy hair.
This change in shape also occurs due to different genetic variations in different groups of people. Straight hair in Asians is due to a variation in two genes, genes different from the ones that influence hair texture in Europeans and Africans.
This affects the shape of the follicle of the hair, which in turn affects the shape of the hair itself.
People with curly hair have asymmetrical follicles, which grow oval hair at a sharp angle that curls. People with straight hair have symmetrical follicles that grow round hair rather than oval hair.
"What makes hair appear a certain way? When they’re actually affected at the follicle level. A hair follicle is a tiny organ beneath the skin that produces hair. For hair to grow, it has to be forced through a tube whose shape affects the hair. People with curly hair have asymmetrical follicles…which grow oval hair that curls."