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Washing your hair properly is probably the most important aspect of hair care; it's key to keeping your hair bright, shiny, strong, and healthy, and washing your hair improperly can seriously affect how it looks, how easy it is to style, and can even exacerbate breaking and split ends.
How sad, then, that people have so many misapprehensions about the process. Misapprehensions, it must be noted, carefully developed and cultivated by cosmetics companies, which they can freely do in an industry with very little oversight. Claims made on shampoo bottles are terribly exaggerated, even blatantly untrue, and supported, if at all, by scientific 'studies' performed by the company and carefully designed to give exactly the results desired by marketing departments. This writeup is intended to give a simple overview of the components of a shampoo and address a few myths associated with them.
ingredients
Water is the primary ingredient of any hair product. It is always first on the ingredients list, and comprises the majority of the product. Some shampoos will list 'spring water', 'purified water', and other such nonsense. Water is water.
There is a pretty good variety of detergents available. Sodium lauryl sulfate, sodium laureth sulfate, ammonium lauryl sulfate, ammonium laureth sulfate, TEA-lauryl sulfate, TEA-laureth sulfate, sodium isethionate, ammonium xylenesulfate, and several others. Some of these ingredients can be irritating or harsh - avoid sodium lauryl sulfate and ammonium xylenesulfate, and there are some controversial, but relevant, concerns regarding both TEA chemicals (particularly in combination with certain preservatives) - and they can also be irritating or cause allergic reactions. I avoid all of the questionable ones.
Other than those concerns, detergents are detergents. Essentially all shampoos use the above, or very similar ingredients, and they come from the same chemical manufacturers. They are all Cosmetic Grade and there is no difference between the detergents used in expensive products and cheap ones.
Lather is associated with cleaning, even though it plays no role in the process. Since some of these detergents do not lather well on their own, separate ingredients like cocamide MEA are added to shampoos to achieve this effect. They serve no purpose besides making the shampoo foamy. Once again, there is a small range of chemicals used for this purpose, and in many cases, one single manufacturer supplies each particular chemical to every company that uses it in its products.
Shampoos and conditioners would both be watery, separated liquids without thickeners added to create the desired consistency. There is a huge variety of chemicals used for this process; common ones are cetyl alcohol, stearyl alcohol, xanthan gum, and even sodium chloride (table salt). While there are many different chemicals used here (unlike the other major components of shampoo), they are not responsible for the product's cleaning function (although they are a necessary part of a product's formulation). Which thickening chemical is used, in general, has no effect on the product's efficacy.
Various conditioning agents are used in most shampoos; common ones include panthenol, various proteins, and silicones like dimethicone and cyclomethicone. 2-in-1 (shampoo and conditioner) products generally use heavy amounts of silicones in their products, as they provide a pleasing conditioning effect and don't immediately rinse away when the shampoo is rinsed off. Once again, the same chemicals are used in both cheap and expensive shampoos; they are all cheap to the manufacturer and they are all available to any cosmetics company that wishes to use them in their formulation.
These are not generally highly touted on the label, although they are essential for shampoo's safety. They prevent bacteria and fungi from growing in the warm, moist product (which is FULL of nutritious organic chemicals.) Even products touted for their 'natural' ingredients use them - and rightly so. Infections can easily occur due to contaminated products and your health depends on their presence. They include methylparaben, propylparaben, diazolidinyl urea, and others.
According to Consumer Reports, this is the single biggest factor that determines consumer approval of shampoos. Clearly they provide no cleaning benefit (whether synthetic or in the form of essential oils, addressed below) but certainly they are pleasant to smell. They're also the most likely ingredients to provoke an allergic reaction.
'Botanicals', vitamins, essential oils, sunscreens, plant extracts, and many other highly-touted ingredients in shampoo do nothing useful for your hair. Vitamins cannot be absorbed by the skin, much less by the hair (being dead tissue), and to be made useful in the body, they must be digested and combined with other substances. Vitamins are coenzymes, meaning that they must be used in association with enzymes, in complex biological processes. They do not have the magical properties shampoo ads would have you believe they do, and what's more, they are generally present in tiny, tiny quantities in cosmetics.
Plant extracts are worse than useless in shampoos. None of them have been shown to have a positive effect on the hair; they are chosen, for the most part, for their fascinating, exotic-sounding names. Until there is real evidence - as in, published, peer-reviewed scientific research, don't be swayed by the Madagascar Honey-Orchid or the Tierra del Fuego Spiny Fire Orange, no matter how fascinating they may sound. Even more damning: read the ingredients list on your shampoo bottle - plant extracts are almost always towards the end of the list, meaning they are present in only the tiniest quantities. On the other hand, some shampoo bottles list them first, referring to "Aqueous extracts of . . . ". Don't be fooled. This means that, in producing the product, they add a tiny amount of plant material to the water prior to adding other ingredients.
Essential oils are another much-hyped ingredient in hair products. I won't take a stand on the usefulness of aromatherapy - but I will state my opinion that whatever validity it has as a medicinal technique, it seems unlikely that a shampoo is a good way to obtain it. Essential oils are an extremely common source of allergies and skin sensitivity. Their only real function in the shampoo is as fragrance.
The worst feature of these plant derivatives is that their presence necessitates a greater amount of preservatives to prevent the product from rotting. Plant extracts are obviously prime food for bacteria and fungi - and so the seemingly 'natural' cosmetics may contain more of the more suspect chemicals to make up for the useless plant material they contain.
don't believe the hype
price
All products are priced at the price the market will bear. Expensive cosmetics use the same chemicals, in the same grade, in the same concentrations. There is a fairly small number of chemicals used in shampoos, and they fall into the categories noted above. None of them is particularly pricey, and the formulations of products are extremely similar from brand to brand. Expensive brands are often owned by the same parent companies as cheaper ones; often, the only difference between an expensive product and a cheap one is the amount spent on marketing and how spiffy the bottle looks. Shampoos cost pennies to manufacture, whether they're dollar-and-a-quarter bottles of Suave or tiny tubes from Aveda or Bumble & Bumble that retail for $20 apiece.
That's not to say that all shampoos are the same. There are well-formulated shampoos, and poorly formulated shampoos. Expensive products may be wonderful, or they may be terrible, and the same is true at the other end of the spectrum. There are no secrets in cosmetics; everything the shampoo contains is divulged on its ingredients list. If one company held a magical cure-all, that really would boost shine by 82% and strength by five times, every other company on the market would have the same product.
the myth of 'natural'
'Natural' is a meaningless term in the world of cosmetics, for reasons we'll discuss later. Suffice it to say that truly natural hair care products - not produced from synthetic chemicals - are few and far between. You will never find them at a drug store or grocery store, much less at a salon. The only effective alternative to detergent is soap, which can be made in a simple products from natural ingredients. It will likely strip the oils from your hair, and leave it dry and unpleasant, while irritating your scalp. Is that surprising? One of the ingredients in soap is lye - sodium hydroxide - a perfectly natural, immensely caustic alkali. Soap is far more alkaline then any hair product should be; bases cause the hair shaft to swell, damaging the cuticle, making hair less shiny, and causing it to be impossible to style.
The 'natural' products you find in most stores have nothing distinct from any other brand. They may contain tiny quantities of plant extracts, but they still clean your hair with detergents. Read the list of ingredients on your favorite natural shampoo sometime. They all still start with the basics: detergents, lather builders, and conditioning agents, and they all come from decidedly unnatural sources.
My references for the above work are largely the books of Paula Begoun, particularly Don't Go Shopping for Hair-Care Products Without Me and The Beauty Bible. | 
