All hair is dead. This isn't a value judgment -- it's just a fact. Hair is composed of protein cells that die before they ever see the light of day. Any claim that a product can nourish or permanently repair hair is false, because you can't revive a dead cell. The differences you see in your hair after shampooing and conditioning are purely cosmetic and short-lived. But don't despair -- there are lots of temporary fixes that work well.
A hair shaft is composed of three concentric layers: the innermost layer is the medulla, which is not affected by hair care products or processes (and isn't even always present); the middle layer is the cortex (the pigment is located here), which can be modified somewhat by dyeing, bleaching, perming and straightening; and the outer layer is the cuticle, which comes into play on a daily basis.
The cuticle is made up of tiny overlapping scales that protect the cortex. It's the condition of these scales that primarily determines whether or not you're going to have a bad hair day. Cuticles that are in good shape are unbroken and lie flat, making hair look shiny and smooth. It's when the cuticles and cortex get damaged that problems such as breakage, split ends and a dried-out look arise.