The structure of hair generally consists of three different layers: cuticle, medulla and cortex.
Cuticle is the outermost layer of the hair and it works to protect the underlying layers from dryness and damage. Cuticle can be damaged by mechanical friction, as when you rebonding your hair, as well as the use of alkaline materials such as shampoo and harsh color.
The second layer, cortex. Mainly consists of hair pigment and air cavity. Cortical layer is a soft layer, so that can easily damaged by chemicals applied to the hair. All of the treatment processes that use chemical working on this layer, including hair dying.
The third layer is the innermost layer is called medulla. This layer only found on thick hair, while in the blonde hair and thin hair medulla does not exist.

Factors Can Cause Damage To The Hair

Environmental Factors

Air pollution and sun are the two big factors. Though there isn't much we can really do to prevent these factors from challenging our hair, we can try our best to keep our hair covered when we are out in the elements. It's important for your hair to keep it covered with a hat, scarf, or whatever is fashionable to you, if you are going to exposed to the sun for long periods of time.

Chemical & Mechanical Factors

Process of hair curling, this pertains to daily heat abuse through the use of flat irons, curling irons or pressing combs. It also applies to high temperatures used even once. Hair can and will burn if extremely hot tools are used on it.
Applying harsh color on chemically processed hair is another potential damage scenario. The lifting and depositing process on hair that's already processed is too harsh to leave in the hands of anyone who doesn't have experience treating double-processed hair.
Smoking is also something that can cause your hair to look dull, frizzy, or even grow slower.

Vitamins For The Healthy Hair

There are many vitamins and minerals you can take that can also help your hair. Most of us don't eat enough vitamin rich foods in our daily diets and that can really have an effect on our appearance. If your hair is looking dull, damaged, or lifeless, you might consider taking daily vitamins.
These are vitamins and minerals needed by the body in order to make hair looks beautiful and healthy.
Vitamin Bs, B12 & B6. B vitamins is for hair nourishing and good for making hair strong and thicker.
Inositol along with Folic acid are other vitamins needs for healthy hair growth.
Beta Carotene also very important as to help hair well grow and help protect it from the environment and radical effects.
Magnesium, sulfur and zinc are play a part in keeping your hair healthy and looking shiny and beautiful.