This is a photo of my Mom frying zuchinni squash, a very wonder-filled treat! Why does cooking in oil make things crispy, while cooking in water makes them soggy?
To make foods crispy, the idea is to remove water. Think about bread going stale. The hard texture is from loss of water. Hot oil heats the water in the surface of the food to the point where it boils away. If the oil is hot enough, the pressure of this water vapor keeps the oil from saturating the food. If the oil is not hot enough, the food will be oily. If the oil is too hot, it will start to break down, which gives it a bad flavor and smell.
Different oils work best at different temperatures. The key to frying is to have the right kind of oil at the right temperature.