Storm windows are most commonly found mounted on the outside (but can be on the inside as well) of a primary window meant to improve the overall thermal efficiency, and sometimes allow for a screening/ventilation element which can be permanent or temporary.
Storm windows commonly come glazed with glass panes or plastic panes (acrylic or polycarbonate). Storm windows add a thermal advantage (improved U-Rating) similar to insulated glazing, where the added pane and frame provide extra resistance to wind, rain, and vapor passage especially coupled with single paned primary units. Lastly, they also offer resistance to damage of the primary window in the event of inclement weather or other unintended abuse. Storm windows are used much less frequently in areas where climate is hot or insulated glass already exists. Many home owners and contractors opt to install storm windows in addition to existing windows, instead of installing replacement windows. Storm windows may be caulked on the head and jambs, but shouldn’t be caulked at the sill to allow for water to escape.