Well, the basic understanding I've garnered (as far as perused documents online and in more than a few diet/nutrition books) is that fat is a miracle calorie. Denser than protein or carbohydrate, it will sit in your stomach and keep you fuller longer. How that fat is used within the body all depends on the host's diet, activity level, etc. It can be a superior energy source, or excess baggage.

Saturated fat, in particular, serves no purpose in the body other than storage (excluding MCT's, a uniquely architectured saturated fat derived from coconut oil). Transfat simply cannot be metabolized and should be utterly avoided. Mono and polyunsaturated fats are mostly neutral and can be mobilized easily under the proper conditions (the same goes with the above with hard enough work). However, a high enough insulin spike impairs fat mobilization utterly, rather, it signals the body to store it (ie; a couple tablespoons of natural peanut butter is considered a healthy fat, but when it's slathered on enriched Wonderbread and sandwiched with sugary grape jelly, it'll rape your thighs).

So, if fat is to be ingested, best it be accompanied by quality (or tagalong) protein and complex carbs (or no carbs at all).

In summation, I'm not really telling you anything you can't learn in McDonald's UD2.0