Perhaps you've heard most of them already:
saturated, monounsaturated, polyunsaturated, rancidity, and smoke point. Some of these terms refer directly to a fat's molecular structure; others terms refer to an oil's purpose and properties. Although the following chemical terms are important for health reasons, the oil's molecular structure will also impact the finished product. This bare bones chemistry isn't too difficult to grasp, and can be quite helpful when assessing new oil sources. But you can skip these explanations if scientific language makes you break out in a rash. The most important information for the cook is the physical and culinary qualities described in Step 3 and Step 4.

• How is an oil saturated or not? Every oil molecule contains places for hydrogen atoms to hook on to carbon atoms; but be clear from the beginning--the number of attached hydrogen atoms is less important than the number of atoms that could be attached but aren't. What's the importance of hydrogen atom availability? The more available hydrogen slots there are, the less stable the oil is.

• Lack of stability increases the rate of oxidation. Oxidation causes rancidity, which is essentially spoilage due to overexposure to air. Oxidation also creates modern menaces such as free radicals, molecules now known to contribute to cancer and premature aging. (Rancid cooking oil often exudes an "off" aroma: sometimes industrial-smelling, or merely bitter. For example, if what you planned to put in your mouth smells more appropriate for a car engine, toss it.)
  
  
• Saturated fat is a fat with a full parking lot--that is, no room for additional hydrogen. It's solid at room temperature, quite stable (won't oxidize easily), and implicated in high cholesterol counts and serious, potentially fatal diseases. This is the type of fat that health-care professionals are encouraging you to avoid.
  
  
• Monounsaturated fats have just one unfilled slot. They're liquid at room temperature but can solidify when refrigerated. (To re-liquefy, measure the amount that you need and let it sit at room temperature). They are also the darlings of contemporary nutritionists, since monounsaturates are capable of raising the levels of HDLs in the blood stream (HDLs are high-density lipoproteins, which act as cholesterol sweepers), while lowering LDLs (low-density lipoproteins, which are the cholesterol litter bugs).
  
  
• Polyunsaturated fats have two or more available hydrogen slots. They are always liquid, refrigerated or not. Polyunsaturates were hailed in the 1960s for their cholesterol-lowering powers until they were discovered to lower the beneficial HDLs as well.
  
  
• Oils are made of chains of different fat molecules; some molecules are saturated, some aren't. The percentage of each type of fat varies widely, and that explains why some oils are more saturated than others. Oils take their classification from the type of fat that is in the majority. For example, a fat like olive oil is 10% saturated fat, 75% monounsaturated fat, and 15% polyunsaturated fat. So for a little quiz, what category is olive oil in? You guessed it--it's a monounsaturated oil.

Smoke point is the temperature at which an oil or solid fat burns. The higher the smoke point, the higher the heat threshold. Here's the key point to remember: the more polyunsaturated an oil is, the more unstable it is at room temperature--and potentially dangerous when heated.