What’s the difference between monounsaturated fats, polyunsaturated fats, and trans fats? Why are some fats now considered healthy?

Monounsaturated and polyunsaturated fats are found in higher proportions in plants, nuts, seafood and are usually liquid at room temperature. The exceptions are certain tropical plant oils, such as coconut oil, palm oil, and palm kernel oil (which are high in saturated fat) and partially hydrogenated oils (which contain trans fat). (fda.gov)
Like all dietary fats, monounsaturated and polyunsaturated fats specifically help the body absorb certain vitamins, cushion and insulate the body, and support many body processes. • Monounsaturated and polyunsaturated fats contribute vitamin E to the diet. • Polyunsaturated fat is a source of two essential fats. These fats are considered essential because they are required for normal body functioning, but they can’t be made by the body and must be obtained from food. Essential fats play a role in many body processes, including immune and nervous system function, blood clotting, and blood pressure regulation. https://www.accessdata.fda.gov/scripts/InteractiveNutritionFactsLabel/factsheets/Monounsaturated_and_Polyunsaturated_Fat.pdf