An awning window is designed so the bottom of the sash will swing outward from the home when opened. Hoppers, on the other hand, are designed so that the top of the sash will swing inward from the wall when opened.
Awning windows were used before the invention of air conditioning. Because an awning window allows fresh air to flow into a home while keeping light rain out it was a perfect solution.
Hopper windows remain popular today for small spaces such as basement rooms and small bathrooms. Because a hopper window opens completely, unlike a slider or double-hung that only opens half-way, it is able to allow more air ventilation.