When cutting calories, one is always told to minimize fat intake, especially when eating animal parts. Trim the fat off steaks, use a leaner ground meat, remove skin from poultry, etc.
But is all fat created equal? Is, say, the fat that collects around a chicken’s hind quarters the same substance as fat on a steak, or the same that is found in a fatty fish? Or is some fat qualitatively different, or more or less saturated?
According to On Food and Cooking, the fat of beef is the most highly saturated, followed by lamb. Pork and chicken are less so (and are much leaner to begin with). Vegetable fats are lower still, to the point where they’re usually liquid at room temperature. Fish fats are often unsaturated, as well, and a number of them have a double bond at the third and fourth carbon in from the end: the so-called “omega-3” oils, which have several health benefits.
The differences, though, are relatively negligible between the meats; they’re all more saturated than you want as a large portion of your diet.
TimeWinder’s got the gist of what I would respond with. There are hundreds of variants on fats, which include the length of the chain and degree of saturation. The best rule of thumb for the non-chemist is whether the fat/oil is liquid at room temperature or not.
One thing I will add: they’re all the same calorie-wise. Olive oil might be less likely to clog your arteries than lard, but it will clog your hips just the same. You may notice many commercials are now being forced to specify “lighter tasting” when talking about why things like olive oil are supposed to be better.