Hair types are generally categorized into four main groups: straight, wavy, curly, and coily. Each type is defined by the shape of the hair strand and its natural texture. Straight hair lies flat and has a smooth appearance, while wavy hair forms gentle waves. Curly hair has defined curls or ringlets, and coily hair features tight, spring-like curls. Within these categories, hair can also vary in thickness and porosity. Fine hair is thin and delicate, while thick hair is more robust. Porosity refers to how well hair absorbs moisture, affecting its overall health and styling needs.