All plant oils are nearly pure fat and, like all fats, are combinations of saturated, monounsaturated, and polyunsaturated fatty acids. Fatty acid molecules vary in the length of their carbon chain and the degree of saturation (that is, how many hydrogen atoms are attached to the carbon atoms), factors that help determine whether a fat is solid or liquid at room temperature. Plant oils are usually made from seeds (which botanically include nuts), but also from some legumes (like soybeans and peanuts), fruits (like olives and avocados), and vegetables (like corn). They are usually categorized according to their predominant type of fatty acid.
Saturated fatty acids carry all the hydrogen atoms they can hold. Highly saturated fats come chiefly from animal sources, including butter, milk, and meats. Three plant oils—coconut, palm, and palm kernel—are also high in saturated fat.
Unsaturated fatty acids, primarily from plants and fish, do not have all the hydrogen atoms they can carry (that is, the carbon atoms are not “saturated” with hydrogen). If one pair of hydrogen atoms is missing, the fatty acids are called monounsaturated (olive, canola, safflower, avocado, and peanut oils consist largely of monounsaturated fatty acids).
If two or more pairs of hydrogen atoms are missing, these fatty acids are called polyunsaturated (sunflower, flaxseed, walnut, grapeseed, corn, soybean, and cottonseed oils are primarily polyunsaturated). Today, safflower and sunflower oils are often made from seeds bred to contain mostly oleic acid, which is the main monounsaturated fatty acid in olive oil. Sesame oil contains equal amounts of monounsaturated and polyunsaturated fats.
Canola, walnut, soybean, and especially flaxseed oils also contain alpha-linolenic acid (ALA), an omega-3 polyunsaturated fatty acid related to the heart-healthy longer-chain omega-3s found in fish. The body can convert ALA to the longer-chain omega-3s to a limited extent; some research suggests that ALA may also have cardiovascular benefits beyond that.
Trans fats are formed when vegetable oils are partially hydrogenated, a process that adds hydrogen atoms to unsaturated fatty acids, making them more saturated and more solid—and unhealthful.
Also see Is Olive Oil Really That Special?