What is oil and which one should I use in Cooking?

Cooking oil is purified fat of plant or animal origin, which is liquid at room temperature.
Some of the many different kinds of edible vegetable oils include: olive oil, palm oil, soybean oil, canola oil, pumpkin seed oil, corn oil, sunflower oil, safflower oil, peanut oil, grape seed oil, sesame oil, argan oil and rice bran oil. Many other kinds of vegetable oils are also used for cooking.
The generic term "vegetable oil" when used to label a cooking oil product refers to a blend of a variety of oils often based on palm, corn, soybean or sunflower oils.
Oil can be flavoured by immersing aromatic food stuffs such as fresh herbs, peppers and so forth in the oil for an extended period of time. However, care must be taken when using garlic and onions to prevent the growth of Clostridium botulinum (the bacteria that produces toxins that can lead to botulism) in this medium.

Cooking with oils

Heating an oil changes its characteristics. Some oils that are healthy at room temperature can become unhealthy when heated above certain temperatures. When choosing a cooking oil, it is therefore important to note the oil's heat tolerance, and to match the oil to its use in cooking. Oils that are suitable for high temperature frying (above 280°C/500°F) include:

• Almond oil
• Apricot kernel oil
• High-oleic safflower oil or sunflower oil
• Carotino oil
• Peanut oil
• soybean oil

• Carotene oil
• Canola oil
• Walnut oil
• Sunflower oil
• Sesame oil

Whether refined or not, all oils are sensitive to heat, light and exposure to oxygen. Rancid oil has an unpleasant aroma and acrid taste, and its nutrient value is greatly diminished. To delay the development of rancid oil, a blanket of an inert gas, usually nitrogen, is applied to the vapor space in the storage container immediately after production. This is referred to as tank blanketing.
It is best to store all oils in the refrigerator or a cool, dry place. Oils may thicken, but if you let them stand at room temperature they will soon return to liquid. To prevent negative effects of heat and light, take oils out of cold storage just long enough to use them. Refined oils high in monounsaturated fats keep up to a year (if they are olive oil, they'll keep up to a few years), while those high in polyunsaturated fats keep about six months. Extra-virgin and virgin olive oils keep at least 9 months after opening. Other monounsaturated oils keep well up to a high eight months; unrefined polyunsaturated oils only about half as long. (top)

There is a general lack of consensus on the smoke points of many popular oils, as well as a lack of standardization for qualifiers such as "refined". Empirical tests are heavily dependent on the qualities of the particular samples (brand, composition, process) available, but appear to be the major source of available data. In the field, experience trumps references, and there is no source that seems truly authoritative. A crude guide is that lighter, more refined oils have higher smoke points. If there is any doubt at all, be fully prepared to extinguish a burning oil fire before heating.

Proper disposal of used cooking oil is an important waste-management concern. Oil is lighter than water and tends to spread into thin and broad membranes which hinder the oxygenation of water. Because of this, a single liter of oil can contaminate as much as 1 million liters of water. Also, oil can congeal on pipes provoking blockages.
Because of this, cooking oil should never be dumped on the kitchen sink or in the toilet bowl. The proper way to dispose of oil is to put it in a sealed non-recyclable container and discard it with regular garbage. (top)

• Cooking oil can be recycled. It can be used to produce soap and biodiese O'Brien, R.D. (1998). Fats and Oils: Formulating and Processing for Applications. Technomic Publishing Co., Inc.. 
• Potter, N.N. and J.H. Hotchkiss (1995). Food Science - Fifth Edition. Chapman & Hall, 359-80, 402-7. 
• Nutrition: Concepts and Controversies, Sizer and Whitney

• http://en.wikipedia.org/wiki/Cooking_oil