Awning windows are pretty straightforward, and are similar to casement windows. They are like casement windows turned on their side, actually, but with slightly different dimensions. They are usually wider than they are tall (long and rectangular for placement over doors and windows, for instance), but they are available in many shapes and sizes.
Awning windows tend to let in more light than casement windows. They are relatively common in homes that have traditional double-hung windows, because they are so complementary. You will also often find them above picture windows.
Because the hinges open from one fixed side, like casement windows, awning windows use compression seals and are thus very energy efficient. Compression seals provide a tighter seal, and casement and awning windows use compression seals on three sides. Windows that slide, such as gliding or double-hung windows, only use compression seals on two sides.