shampoo strips out the dirt and other stuff in your hair, even the natural hair oils that are essential for healthy hair. conditioner helps rehydrate your hair. you should shampoo twice a week (usually) while conditioning three or more times a week. but if you're not that fussy you could always just opt for 2-1 shampoo, where there's both shampoo and conditioner in the bottle. the downside is that 2-1 shampoos don't do as well as regular shampoo and there could be a buildup of conditioner from using it, so much so that you might still need a proper shampoo to wash out all the accumulated conditioner.