A sweatshirt is serged and a sweater has seams that are finished by weaving them together. This prevents them from unraveling. The back and inside of the sweatshirt is brushed carefully to make it softer and comfier. This is called fleece. A sweater on the other hand, is already comfy and soft by the way it is designed and constructed. In most good quality sweatshirts, the fabric on the inside and outside of the sleeve can be pulled apart. This suggests that it has two layers. A sweater has just one layer throughout. A sweatshirt does not have a front opening, but a sweater may or may not have one depending on the design or style. Styling sweaters are super easy, but so is styling sweatshirts!