Treatment
Treatments are targeted courses of treatment for a specific problem: breakage, color fading, an unbalanced scalp, or porous hair after bleaching. They usually work in a short series rather than as a one-off. Read the instructions carefully, as the order in relation to shampoo and conditioner varies by product.
First, determine what you want to solve, because a treatment that is not suited to your problem will do nothing. If your hair is breaking, look for a bonding or protein treatment. If your color is fading to yellow or orange, a pigment treatment. If your scalp is itchy or flaky, a scalp treatment. If it just feels dry, you don't need a treatment but a mask.
The order really varies by product, and that is not a minor detail. Some go on towel-dried hair before shampoo, others after conditioner, and others again are left in. Used in the wrong order, the product will not work, no matter how good it is.
Count on a series, not a one-time application. Most courses are designed for three to six applications. If you notice nothing after the entire series, it was not the right product — more of the same is a waste of your money.