Technically, you could really cut down on your personal hygiene products and wash your hair with the same soap that you use for your body. A separate soap for hair didn't really exist until the 1860s, when people weren't generally bathing all that often anyway. But shampoo is now very different from regular soap. It contains ingredients designed not only to remove dirt, but also get rid of excess oil, dandruff and buildup from the environment. Shampoo is also supposed to make your hair shiny, smooth, moisturized and manageable.
Even when more frequent bathing became the norm, people still weren't washing their hair frequently. In the early 20th century, it was typical to wash your hair once a month. An article in the New York Times dating from 1908 stated that you could wash your hair every two weeks [source: NPR]. Some point to the 1970s shampoo ads featuring models with shiny, beautiful locks as the inspiration for daily or near-daily hair-washing.
So, we've established that shampoo is a necessity for most people. But unless you have a problem such as dandruff, or a specific hair condition you want to manage, such as protecting dyed hair, it doesn't really matter whether you choose an expensive salon brand or the no-name one. All of them will get your hair clean, and that's the basic idea. More expensive shampoos claim to keep your hair healthy with the addition of vitamins, but keep this in mind: Hair isn't alive, so there's only so much that those vitamins can do for it.