English can be a complicated thing. If it isn't your first language you know this all too well and if you have a friend from another country, you've probably heard them complain. If you look up fat in the dictionary you'll find out it can be used as an adjective (he is fat) or a noun (that food contains a lot of fat). Unfortunately these two meanings get confused and lead people to believe that eating the noun will make you the adjective.
We Need Fat
We need fat to survive. Without it, our bodies can't function correctly. Eating it doesn't necessarily make you fat. What can make you fat is if you eat too much of it. This is true for fat, carbohydrates and even protein. A way to stay healthy and get the delicious flavor that fat adds to food is to differentiate between the good fats and the bad ones.
Types of Fat
There are three main types of fat: saturated, unsaturated and trans fat. Saturated is found mostly in animal products (meat, poultry and dairy) while unsaturated fat comes mainly from plant sources (nuts and oils).
Good vs. Bad Fat
Trans fat is a man made fat that can be found in a wide variety of foods (margarine, baked goods and snacks). While you can eat a food that is primarily made up of one kind of fat, usually foods contain a mixture of saturated and unsaturated fats.
Unsaturated (healthy) fats are the ones that come from plant sources. Most oils, nuts, avocados and other plants are rich in these. Fish are also high in unsaturated fats. Saturated (bad) fats come from animal products such as dairy (milk, cheese, creams) and meat (beef, pork).
Trans (worst kind of fat) fat comes in anything that contains hydrogenated or partially hydrogenated oils. Sources include margarine, baked goods, and junk food. These aren't the only sources so be sure to read the ingredients.
The Bottom Line
All sources of fat contain the same amount of calories (9 calories per gram) so even eating excessive healthy fats can cause weight gain. Choosing the healthy ones won't make you skinnier but will make you healthier.