Your curl type is determined by the shape of the follicle that your hair grows out of from your scalp. The flatter or more oval shaped the follicle, the curlier your hair; while the more circular the cross-section, the straighter your hair.
Your curl pattern is also identified by the shape that the strands of hair make, whether they kink, curve, or wind around themselves into spirals. The sub-classifications of A to C are based on the width or diameter of your wave or curl. As have a wider pattern size, Bs medium, and Cs the smallest of the three. Below is the 411 on everything you need to know about curls and curls classifications.