Saturated fats are usually described as "bad" and polyunsaturates are usually described as "good". This is simplistic. Everything is bad in excess, even polyunsaturates.
The thing about fats is that there are four basic types (saturates, monounsaturates, omega-6 polyunsaturates and omega-3 polyunsaturates), and these must be consumed in roughly the right proportions for optimum health. Suffice it to say, the majority of people in the West do not eat them in anywhere near the right proportions. So, what exactly are fats?
Fats are an ester of glycerol and fatty acids.
1 molecule of glycerol + 3 molecules of fatty acid = 1 molecule of triglyceride (fat) + 3 molecules of water.
It's the fatty acids that determine whether a fat is sat, mono etc. The four different types of fatty acid all have a CH3 at one end and a COOH at the other. The difference is in the middle section.
Saturated fatty acids have a middle section consisting of all CH2's. Here's a diagram for Stearic acid (the predominant fatty acid in beef fat):-
Monounsaturated fatty acids have one C=C bond in the middle, which happens to be 9 from the left-hand end resulting in monounsaturates also being referred to as omega-9's, as omega is at the end of the Greek alphabet. Here's a diagram for Oleic acid (the predominant fatty acid in olive oil):-
Omega-6 polyunsaturated fatty acids have two or more C=C bonds in the middle with the last one being 6 from the left-hand end. Here's a diagram for Linoleic acid (the predominant fatty acid in sunflower oil):-
Omega-3 polyunsaturated fatty acids have three or more C=C bonds in the middle with the last one being 3 from the left-hand end. Here's a diagram for Alpha-linolenic acid (the predominant fatty acid in flax-seed oil):-
These diagrams are a bit misleading. Where there is a C=C bond, there are two H's on the "underside" only of the molecule. This asymmetry causes the H's to repel each other and bend the molecule into a V-shape at each C=C bond. C=C bonds with H's on the same side are known as "cis" bonds. The above molecule is really cis, cis, cis (c,c,c) Alpha-linolenic acid. The other type of C=C bond is known as "trans" and looks like the following diagram:-
This is a diagram of trans, trans, trans (t,t,t) Alpha-linolenic acid. As the H's are on opposite sides of the molecule, they do not repel each other and the molecule is straight as shown above. Note that saturated fatty acid molecules are naturally straight. Therein lies the problem with trans-fatty acids. They're straight like saturated fatty acids but have unsaturated bonds which are prone to oxidation. Click here. WARNING! Heavy-duty organic chemistry!
Our bodies take trans fatty acids and incorporate them into cell membranes as if they were saturated fatty acids. This results in atherogenicity (artery-clogging), damage to the immune system and other health problems. Trans-fatty acids are found in partially-hydrogenated vegetable oils and so any processed foods or cooking/spreading fats which has the word "hydrogenated" high-up in the ingredients list should be avoided. These are bad fats. There are also naturally-occurring trans-fatty acids made by bacteria in the stomachs of ruminant animals, like Conjugated Linoleic Acid (CLA). This looks a bit like the diagram below:-
This has one of the C=C bonds shifted to the left and also has one cis bond and one trans bond, so the molecule is always bent. CLA has certain beneficial properties but medical studies show mixed results. It's certainly not artery-clogging, so don't let anyone put you off eating butter from grass-fed cows (e.g. New Zealand butter) by suggesting that it has nasty trans-fats in it. CLA is a harmless trans-fat.
Anyway, back to diet. I've read that saturated fat consumption should be about 10% of total calories. This is because, even though sat fats are not essential (our bodies can manufacture sat fats), this guarantees adequate levels of sex hormones in the body. I've also read that total polyunsaturate consumption should be about 5% of total calories, with a ratio of omega-6 (O6) to omega-3 (O3) of 2:1. As O3's are only found in greater quantities than O6's in flax-seeds (a.k.a. linseeds) and oily fish, and many people eat way too little or no oily fish (and who, other than body-builders and some vegetarians/vegans, eats flax-seeds?), the O6:O3 ratio in the West is about 20:1. This is due to the widespread consumption of meats, eggs & milk from grain-fed animals, grains, nuts and seeds. So, it's not surprising that there are high rates of heart disease and other inflammatory diseases in the West, as O6's end up in series 1 & 2 prostaglandins and series 2 prostaglandins are pro-inflammatory. O3 fats end up in series 3 prostaglandins, which are anti-inflammatory.
So eat up yer oily fish if you're not vegetarian or vegan. Otherwise, eat up yer ground-up flax-seeds!
Monounsaturates can make up 15% to 35% of total calories, depending on activity levels. Sedentary people on average burn twice as much energy from fats as from carbs. So, if energy from protein is 25% say, 25% of energy can come from carbs and 50% can come from fats i.e. a 2:1 ratio of fats:carbs. When active, more carbs are needed by everyone.
Which fats contain which fatty acids? Click here to see tables of Properties and Composition of Vegetable and Special Oils, and Properties and Composition of Animal and Marine Fats and Oils.
For high-temperature cooking, saturates are the least likely to oxidise (when they're on fire, they're oxidising!), followed by monos, then omega-6's and then omega-3's. An oil doesn't have to be smoking to be oxidising. Alpha-linolenic acid oxidises at room temperature, which is why linseed oil is used to varnish cricket bats and soften putty. The best non-animal fat for high-temperature cooking is therefore Coconut Oil, followed by Palm Oil and then Olive Oil. Extra-virgin Olive Oil has a lower smoking point than refined Olive Oil (due to higher levels of free fatty acids), but has higher levels of polyphenol antioxidants, which is good.