From NuGOwiki

A fatty acid is a carboxylic acid often with a long unbranched aliphatic tail (chain). Non-saturated or unsaturated fatty acids contain double bonds along the chain, which can either exist in a cis or trans configuration. A cis configuration means that the two carbons are on the same side of the double bond. The rigidity of the double bond freezes its conformation and, in the case of the cis isomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. The more double bonds the chain has in the cis configuration, the less flexibility it has. When a chain has many cis bonds, it becomes quite curved in its most accessible conformations. A trans configuration, by contrast, means that the next two carbon atoms are bound to opposite sides of the double bond. As a result, they don't cause the chain to bend much, and their shape is similar to straight saturated fatty acids. In most naturally occurring unsaturated fatty acids, each double bond has 3n carbon atoms after it, for some n, and all are cis bonds. Most fatty acids in the trans configuration (trans fats) are not found in nature and are the result of human processing (eg, hydrogenation). You'll find more on fatty acids on the Wikipedia link.