Appliance of Science: Why do I have curly hair?

Getting to the root of the matter, and untangling the mystery

What's the difference between curly and straight hair? 

When it comes to hair type there are plenty of variations from light waves, ringlets, poker straight, or, my own current state… all over frizz, but we can most easily place hair into two groups, curly or straight. What exactly is the difference between straight and curly hair though, and what causes each?