Lipids or fats are organic substances that provide to the human body lots of energy. By burning just one gram of fats you get around 9 calories of energy.
The Wikipedia definition of lipids is: ”Lipids are broadly defined as any fat-soluble (lipophilic), naturally-occurring molecule, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.”
After their origin, fats can be vegetal or animal and after their structure fats are saturated (the solid ones) or unsaturated fats (the liquid ones like oil). Fats get deposited around the organs and under the skin, creating energy reserves for extreme situations which you body might experience (extreme cold, intense physical exercise). Also fats can travel in the blood stream. The packing of fats over the limit of what your body needs creates obesity. The increase of the quantity of fats in the blood stream leads to hyperlipidemia which in turn can create arteriosclerosis. Arteriosclerosis takes place when the lipids stick to the walls of the arteries.
Besides their energy providing role, fats improve the taste of foods and create the sensation of satiety faster. Fats also stimulate the intestinal contractions and the pancreatic secretion. Some vitamins that are only soluble in fats cannot be absorbed by your body if it lacks lipids.
Normally fats should represent 15% of our total weight. In a balanced nutrition fats should form 20%-30% of all the daily food. The most important quantities of fats are found in vegetable oils(100%), butter and margarine(80-85%), cheese and sour cream(25%), beef(19%), pork meat(19-40%), lard(100%), eggs(10%), milk(4%), nuts and peanuts(40%-50%). In the parenthesis are the percentages of fats found in the aliments compared to their total composition.
For more information also read The good fats and the bad fats short guide.