Shampoos are designed to remove dirt, excess oil and dead skin cells from the scalp and hair. Using shampoo isn't rocket science, but there are one or two things you should know. Unless your hair and scalp are unusually dirty or have lots of buildup, it's only necessary to lather once. Concentrate on the scalp, and don't scrub -- hair is fragile when wet. And most important, be sure to rinse well.
There are various cleansers -- called "surfactants" -- used in shampoo; among the most common are sodium or ammonium laureth sulfate, sodium or ammonium lauryl sulfate, and TEA-lauryl sulfate. Of these, sodium lauryl sulfate and TEA-lauryl sulfate tend to be somewhat harsh and can irritate a sensitive scalp. Most shampoos (except clarifying shampoos) also contain some conditioning agents like humectants, silicone and protein, but in much lower amounts than in conditioner; these stay on your hair until the next time you wash it. Other ingredients produce lather, make the product thick and give it a nice scent and appearance.
Conditioners are made to protect hair from heat and styling damage; reduce friction both between individual hairs and between hair and your comb; moisturize; reduce static and porosity; and impart shine. Generally, the longer you leave a conditioner on, the more chance it has to attach itself to the cuticle (it doesn't penetrate all the way into the cortex);deep conditioning with heat is the most effective. But again, rinsing well is important (unless you use a leave-in conditioner).
There's a long list of ingredients that are used in conditioners. Humectants like glycerin, panthenol and quaternium-22 bind water to the hair. Silicones (ingredients ending in the word "methicone") add shine and lubricate. Proteins fill in chips in the cuticle, making hair feel more full. And emollients, such as fatty acids and fatty alcohols (cetyl alcohol, stearyl alcohol and stearic acid are a few), oils and lanolin coat the hair to protect it and seal in water, though oils and lanolin can be too greasy for many people.