Your hair strands are made up of overlapping protein-based keratin scales that can lift open and seal shut. These scales protect the inner layer of your hair which is called the cortex. The cortex is made of a string of proteins and is what gives hair elasticity and strength, which is why hair that is badly damaged cannot return to its original shape and snaps when stretched.
Damaged hair looks
damaged because the scales stand up and outward exposing the cortex, allowing for harmful substances to be deposited, making your hair feel dryer and as a result - harder to brush. Damaged hair is often refered to as porous
hair. To obtain the appearance of healthy hair (I use the term appearance here because all hair is dead) it is important to understand two things - how to control the opening and closing of your hair's cuticle layer and how to obtain and maintain the desired level of moisture. One analogy I like is that of a rubber band, how many times can you stretch it and release before it snaps? Its the same with hair strands; when hair is washed water is absorbed through the scales and into the cortex, causing the strand to expand, and when the water evaporates and your hair dries your hair strand contracts. Hair strands can only take so much of this before they snap. It's for this reason that people use pre-poos to protect their cuticle layers during washing and prevent excessive moisture loss. All hair care products are geared towards protecting, maintaining and substituting the cuticle layers of your hair.