Hoodies are pieces of thicker clothing with a hood component that you can throw over your head. They are generally less formal than a sweatshirt, which is a long sleeve garment that you can wear in a multitude of situations. Sweatshirts are thicker than t-shirts but do not usually come with a hood. A pullover occupies somewhat more vague terrain. It can be a hoodie or a sweatshirt because its one defining quality is that it is non-zippered, meaning you have to pull it over your head to wear it.