Whether you're the type of baker who haphazardly shovels all the ingredients together or precisely measures all components into little ramekins and bowls before you even pre-heat the oven, there comes a time when you might be missing a key ingredient. That's when the only choice you have is to either go knocking on your neighbor's door or devise some sort of emergency substitution. Baking is a science, as they say. So replacing any ingredient with another is an experiment, often with indeterminable results.
Sometime you just want to replace an ingredient to make a recipe a little healthier. Applesauce has been a common substitution for oil in baked goods for some time. The typical ratio to substitute is 1:1, which means if the recipe calls for 1 cup oil, feel free to substitute 1 cup applesauce. But the texture of the baked good will turn out different--slightly more cakey. It might not be as noticeable in muffins or quick breads, but in a brownie recipe, for example, the brownies would not be as fudgey, but more light and airy. We recommend you replace only about 75% of the oil with applesauce. That little bit of oil does make a difference in the final texture.
Typically though, it's not a good idea to make overarching substitutions, because a greatly altered recipe will most likely result in failure. For example, you can't substitute yeast for baking powder even though both are leaveners. They work in entirely different ways. You can however configure a quick bread or cookie recipe to your liking very easily. If the recipe calls for walnuts, you can pretty much substitute any nut. If it calls for raisins, you can pretty much replace it with any dried fruit as long as it's equal in size.