What pgpeanut says is on the money, but there's an exception: trans-fat would be solid at room temperature. Only unsaturated fatty acids can be trans-fatty acids. As was stated, saturated fatty acids have a straight shape while unsaturated fatty acids have one or more bends in the molecule. However, there's more than 1 way the the fatty acid can bend. The shape you find in nature is cis-fatty acid. Bend it the other way and it's trans-fatty acid, which always has a much higher melting point and can be solid even though it's unsaturated.
Oleic acid is common in olive oil and most oils. It's liquid as a free acid or as a fat. Trans-oleic acid (aka eliadic acid) melts at about 113 F and is a solid.
Purified trans-fatty acid can be obtained as a free acid. You can't really obtain pure trans-fat as a triglyceride, but as many people have learned, some amount of trans-fat is created in the process of hydrogenating oil to solidify it. It used to be common in vegetable shortening, margarine and the like, but the process of making many of those products has been changed due to health concerns.
BTW, the opposite -- saturated fatty acid that's liquid -- is easy to get. That's what fractionated coconut oil is.