What defines a healthy fat?
Monounsaturated & polyunsaturated fats are the good fats. Saturated fats and trans saturated fats are the naughty ones. When talking about good and bad fats the issue is how they are processed the cholesterol each type of fat provides.
Monounsaturated & polyunsaturated fats increase HDL(good cholesterol) and lower LDL(bad cholesterol).
MUSF's are found in things like meats, whole milk, nuts, avacados, or extra virgin olive oil.
PUSF's are found in fish, nuts, seeds.
Trans-saturated fats are bad (they raise LDL and lower HDL). They are found in things like butter, some oils, and peanut butter. Trans fat is a chemically modified type of fat used to increase shelf life of certain products.
That said the body requires fat in the diet to function properly and you won't die from eating bad fats but too much bad fats in one's diet over long periods of time can greatly contribute to health problems. Heart attacks and strokes for example.
**** Edit: I have amended my original statement to say that saturated fats (Cis-saturated fats) are
not a member of the bad fats club.