Fat is incredibly calorie dense. That means that per gram it has more calories in it than either carbohydrates, protein or even alcohol. Fat contains 9 calories per gram, so if you ate a meal that contained 10 grams of fat, the fat in the meal would make up 90 calories.
There are various types of fats. Not all fats are good for you and not all are bad.
Saturated fats tend to be harmful as they increase blood cholesterol levels. They are primarily found in foods from animal sources, such as meat and dairy products. However they are also found in some vegetable oils, such as coconut and palm oils (tropical oils). These fats tend to be solid at room temperature (e.g. butter).
Polyunsaturated fat tends to lower blood cholesterol levels and contain healthy essential fatty acids (omega 3’s and omega 6’s). They are mainly found in vegetables, although small amounts are found in animal foods such as oily fish. Oils include safflower, sunflower, soybean, corn, cottonseed, olive, canola and peanut, as well as some plant foods such as avocado. They tend to be liquid at room temperature (e.g. oils).
These are partially hydrogenated vegetable oils. Trans-fatty acids are formed when hydrogen is bubbled through vegetable oils to make a semi-solid (think margarine) to prolong shelf life. Some research suggests that trans-fatty acids take on the dangerous properties of saturated fats. Trans-fatty acids are found in processed foods such as baked goods (biscuits, cake, crisps, etc.) and fried food.
Ideally, you should be getting most of your daily fat intake from lean meats (e.g. grilled chicken) and fish, as well as from sources of unsaturated fats (e.g. nuts, olive oil, flaxseed oil). As a rule of thumb 15 –25% of your total calories should be in the form of dietary fat.