Using a bar soap on our head scalp for cleansing hair is nothing new. A lot of people from third world country do just that and some of them have the thickest hair I have ever seen.
1. Since bar soap have harsher chemicals used in making them, it cleanse your head scalp, with a more powerful cleansing action. It is great for people with daily oily hair and scalp, since it easily removes those unwanted extra natural hair oil from scalp.
It is bad for people with dry hair because it makes the hair even more drier and brittle causing your hair to be weak and dry all the time. Having a dry head scalp also leads to dry flaky skin coming off your head scalp and also causes dandruff.
2. Shampoo are manufactured with a mild cleansing action power and are more gentle on your hair scalp. Look for shampoo which has a PH value of 5.5 because that is the closest to the skin PH level you can get.
People with dry brittle hair will benefit from using shampoo to cleanse hair because it is much milder than using a bar soap. It also doesn't cause dandruff and flaky skin scalp.
However, people whose head scalp secretes lots of natural hair oil sebum, will find shampoo not as effective in cleansing their hair.
3. Stay away from hair conditioner at all times because from my personal experience, they do nothing but makes my hair thin and weak. Even some hair expert advises against using any hair conditioner at all.
4. Soap shampoo is probably the newest trend that people are accepting and using in a mass quantity recently. It is basically shampoo made in the shape and solidness of a bar soap. It has all the mildness strength of an actual shampoo while still maintaining shape, size and quality of a bar soap. This soap shampoo aren't cheap and can be as expensive as shampoo and they don't last as long as shampoo does, in terms of how quick it gets used up daily.