The structure of hair

You can find better explanations if you look elsewhere on the web, but here is my cliff notes version of the structure of hair.
The inside of the hair is the cortex.  The cortex is filled with strands of keratin protein.  The proteins form long stretchy rubber band like strands.  These strands give hair it's strength and help support waves and curls.  Fine hair (small diameter of a single hair) is always lacking in protein because the cortex is smaller and can't hold the proper amount of protein easily.  Coarse hair (large diameter of a single hair) has enough protein and and adding more protein (in hair products) can be too much protein.  More on this in later posts.

The outer layer of hair is the cuticle.  The cuticle is like scales on fish or shingles on a roof.  It is actually 4-12 layers of fish scales or shingles on a roof (estimates vary and it varies by ethnicity).  The closed cuticle keeps the proper balance of moisture and protein inside the cortex.  The "shingles" laying flat (closed) also reflects light and make hair shiny.  When the cuticle becomes damaged from heat, sun, wind, brushing, dyeing, straightening, perming, etc., you may lose a layer or more of "shingles", the "scales" will open up, or it may loose some individual "shingles".  You may even get a hole in you cuticle.  Through these openings, the moisture and maybe some of the protein will escape.  When this happens, you hair is not as healthy.  Again, more on this later.  The picture (above left) is of damaged hair under a microscope.

If you looked at a cross section of hair, very straight hair would be a perfect circle like looking at the top of a straw.  The curlier hair gets, the more egg shaped the cross section gets. It is like when you squeeze a straw between your fingers.  Kinky curly hair, usually found in African Americans, is almost flat.  Kinky curly hair not only has C shaped bends like curly hair, it also has torsion twists.  Now try to envision shingling these hair shapes.  The straight, circular based straw would have the "shingles" lie flat against the straw.  The flattened oval shaped kinky curly hair with C shaped bends and twists would have shingles that did not lie flat against the base.  The curlier your hair is, the more likely for your "shingles" to be open (a.k.a. porous).  If your hair is porous, you need to replace the moisture that has escaped and close the openings in the cuticle using protein.  (more on this in later posts)  The structure of curly hair causes it to have very different needs then the needs of straight hair. Curly hair must be treated differently to keep it healthy.
For further reading on hair structure see these random links I have bookmarked:
Exploratorium Magazine, Better Hair Through Chemestry 
Black Girl Long Hair- Sturcture of Hair Close Up