Healthy fat or monounsaturated fat. Use this for cooking and for regular consumption. Examples: non tropical plant based fat. Think olive oil, canola oil, avocado oil, safflower oil, corn oil, peanut oil, sesame oil, peanut butter, avocados. These can decrease your bad cholesterol. They are also high in vitamin E, an essential vitamin. Do pay attention to smoke point to preserve flavor.
Saturated fats : the most controversial one. Saturated fat can increase your bad cholesterol. Saturated fat comes from animal fat and tropical plant based oil like coconut oil, palm oil. Lard, ghee, milk, cheese are all examples of this. Good amount of evidence shows saturated fat can increase bad cholesterol, so don't buy into people telling you otherwise. Limit use of saturated fat( for example ensure no more than 13 gm of saturated fat in a typical 2000 Calorie diet).
To give you some perspective, a cup of whole milk has 4.5 gm saturated fat, an egg has 1.6 gm saturated fat and an 8 oz steak has 19 gms of saturated fat.
Last but not the least is trans fat, the most evil of it all. In 2013 FDA determined that partially hydrogenated fat is not "generally recognized as safe for human consumption." Since then it has been banned in various states like New York and California, but still widely used across in the processed food industry.
Trans fat is favored by the food industry as it is inexpensive, and can be used multiple times to deep fry in the industrial scale.
It can be found in donuts, frozen pizza, pie crusts, cakes, cookies, fries, crackers, biscuits, baked good, stick margarine etc.
It can increase bad cholesterol, decrease good cholesterol and increase risk of heart diseases.
How to avoid trans fat? Read labels. Ensure it says zero gram trans fat or read ingredients to ensure there is no mention of partially hydrogenated oil.
Also remember, whatever fat you consume comes with calories (9 calories per gram), so use in moderation to avoid weight gain. Don't forget to balance your diet.