Protein Part 1- What it does, Link for protein haters, Which ingredients are protein?

Protein does two main things for hair.  1)  It shores up the hair's inner structure, making it stretchy.  This makes hair stronger.  There are long chains of protein in the cortex that stretch like rubber bands.  Adding protein to hair helps keep these chains in good repair.  2)  Hair that is damaged is damaged permanently, but protein can go a long way toward temporarily repairing that damage.  Protein temporarily fills in the openings/holes in the hair's cuticle.  Having the "holes" closed helps hair hang on to moisture and protein inside the cortex.  Fixing the "holes" makes hair shinier, and makes it act healthier.  Curls will bounce up after a protein treatment if you need one.

Protein needs to be kept in balance with moisture.  Too much of either protein or moisture will give you hard brittle hair.  Not enough moisture will give you dry, rough hair.  Not enough protein your hair will be limp.   I'll do a separate post on how to diagnose hair.

I highly suggest you read this article, if you haven't yet (it was linked in another posts).  It explains protein and moisture balance in hair.
The Fine Art of Protein and Moisture Balancing for Black Hair Care.
The article is about African American hair, but the information is true for all hair.

So does this mean you should run out and put some protein in your hair?  No.  Protein isn't friends with all types of hair.  If your hair is non porous, you do not need protein (unless you have fine hair).  In non porous hair the cuticle lies flat and does non need to be repaired.  If you have coarse hair, you do not need protein (unless also porous, then proceed with caution).  Coarse hair has enough protein in its cortex already.  Adding more protein to either of these hair types will give you hard, rough, brittle hair.  Some people have bad reactions to very small amounts of protein in hair products.  These people are protein sensitive.  See the protein sensitive board on naturallycurly.com if you think this is you.

Protein is a miraculous thing for hair that needs it.  It adds bounce, shine, strength, can help with frizz, and make hair act healthier.  Porous hair needs protein.  (use caution if you have coarse, porous hair).  Protein will help temporarily fix the damage done to your hair.  Fine hair tends to like protein, no matter what the porosity.  There isn't much room inside fine hair's cortex, because it is relatively smaller then the cortex of medium or coarse hair.  Fine hair seems to always be lacking protein in its interior.  If your hair needs protein check out the Protein Lovers Forum of Naturallycurly.com.

This link (posted previously) has pictures of curly hair needing protein and hair with too much protein.
The Pittsburgh Curly:  Protein vs. Moisture

Protein ingredients usually have protein in their name.  Some exceptions are amino acids (very small, mild "proteins"), collagen, and keratin.  Protein can be ingredients in styling products, leave in conditioners, conditioners, etc.  It takes time for protein to make its way into the hair's cortex.  It is more effective in things that are left on the hair  (gels, leave in conditioner, etc.).  It is least effective in shampoo, which is only on the hair a minute.  The single most effective way to get protein into hair is with a protein treatment.  Follow a protein treatment with a deep treatment or a moisturizing conditioner to balance out the protein.  More on this in upcoming posts on protein.