Hair is held together (and in place) by hydrogen bonds. These hydrogen bonds are a kind of glue that dictates the shape of your hair.
To change the shape of your hair, you must be able to loosen the hydrogen bonds, which can be accomplished with either heat, or water.
When these are loosened, you are able to rearrange your hair to suit your style before your hair reforms into that shape.
A good Hair Straightener is able to heat your hair up enough to break down the hydrogen bonds so you can straighten, curl or flip out some beautiful waves!
You need to know the best hair straightener for your hair type. Some hair straighteners have adjustable heat controls, which allows you to increase the heat if you have thick hair, or decrease the heat if you have thin/regularly treated hair.