A casement window is a window that, using a crank or lever, opens like a door. Older casements swing on hinges while newer ones have pivot mechanisms. Most swing outward, but models that swing inward are available.
Casements provide excellent ventilation because the entire window, or both halves of it, open. Because they seal tightly when closed, they are considered energy efficient. Another advantage of casements is they are easier to clean than most other windows because both inside and outside surfaces are accessible from indoors.