Cooking oil is different from more expensive oils such as those made from walnuts, almonds or sesame seeds, which are usually added to finish a dish so they impart a distinctive flavour and aroma.
What is it? Any edible oil with a high smoke point - the temperature at which the oil becomes overheated and starts to smoke and break down. If you take it past the smoke point, it can reach the flash point and burst into flames - and oil and fire is not a good combination. Most cooking oils come from vegetables and plants that are inexpensive and easy to grow, such as peanuts, corn, canola and soybeans.
What are the differences? Not much, if you're only considering their ability to fry or saute. Some have higher amounts of saturated fats (the bad stuff) while others are higher in monounsaturated or polyunsaturated fats (the good stuff). Other than that, cooking oils are usually chosen for their flavour - or lack of it. Grapeseed oil, for instance, is the preferred oil for many chefs when they're making infused oils and some salad dressings due to its clean taste - it doesn't add flavour to the infusion in the same way a peppery extra-virgin olive oil would. Some people prefer peanut oil because of the flavour it imparts.
What else? Oils, which are liquid at room temperature, can be made into fats (which are solid or semi-solid at room temperature) through a process called hydrogenation, which, in addition to making them firm and white, also turns the monounsaturated or polyunsaturated fats into saturated fat.
How to use: for frying, oils are usually heated to between 180 and 190 degrees Celsius. If the oil is cooler than that, it will be absorbed by the food, making it heavy and greasy. If the oil is too hot, it will burn the food before it's fully cooked. To keep the oil at its optimal frying temperature, heat it slightly higher, because when you add the food the temperature will drop. Also, avoid adding too much food to the oil at one time, and don't crowd the pan.
Despite the general term, cooking oils aren't always used for cooking. You can make infused oils with a neutral-tasting oil (such as grapeseed) by adding strands of lemon or orange zest, minced fresh herbs or dried herbs, fried garlic or shallots, or (if you're lucky enough to have any) sliced fresh black truffles.
