There are many different types of coveralls available to workers, but generally speaking, coveralls are a single garment worn over a worker's personal clothing in order to protect the worker's front, back, arms, and legs from injury. As opposed to separate shirts and pants, continuous coverage prevents sparks and spatter from entering at your waist, as opposed to separate shirts and pants.
Overalls (also called bib overalls) are clothing items that cover the torso as well as the legs, the torso being sleeveless, and the legs being covered by pockets. Aside from being cooler than coveralls, they can also be worn over shirts and jackets to keep you cool.
As a result of their insulation, these garments will keep workers warm even in cold conditions, such as outside in the winter and in freezers. Overalls are sleeveless garments that cover the legs and upper torso, generally covering both of them. This is not only because they are cooler than coveralls, but they can also be layered with a shirt or jacket if you wish.