Overalls are work clothes designed to go over other clothes to protect them from damage.
Bib-overalls are the most likely the type of clothing to be meant when refereed to as overalls. They will not fully protect the wearer's shirt but will protect the front of the shirt from most spills, as well as any covered pants, if worn. Designs often include extra bib pockets, and loops for tools, and are typically in addition to the shoulder-straps also fastened with side-buttons at the hips.
Shortalls are a variant of bib-overalls that have short rather than long legs. The choice to wear them over bib-overalls is normally the same consideration on wearing shorts; namely some adults might chose to wear them due to heat. But shortalls also have a strong association as children's clothing, where child's play will often damage pants that cover the knees before the outfit is outgrown, hence not worth the extra cost of the materials.
Coveralls, often also called boilersuit or jumpsuit, are designed to cover all the wearer's clothing, including sleeves.