Casement windows open with cranks or levers that open them like doors. 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 both halves of the window 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.