There are actually two types of unsaturated fat, monounsaturated and polyunsaturated.
Monounsaturated fat has one atomic double-bond in it; Polyunsaturated has multiple double-bonds. Of the two, monounsaturated is currently (2005) believed to be healthier, and to actually help lower cholesterol in our bodies.
To make unsaturated Fats stay solid at room temperature, you have to hydrogenate them by break the double-bonds and attaching hydrogen molecules where the double-bonds were. This makes it into a fat called “trans-fat.” Many people now consider this the worst kind of fat for our body — our body doesn’t know how to process it, because it didn’t exist in nature.
For the most part, unsaturated fat come from plant material such as olives.
All fats contain a combination of saturated and unsaturated fats in varying proportions.
In terms of how fattening Unsaturated Fat is, is just as fattening as saturated fat: 120 calories per tablespoon.