The most important information about a lipid one needs to know relate to its carbons and double bonds. Thus, one of the reference systems for categorizing lipids tries to summarize this information neatly and efficiently. An example looks like this...

18:1 Δ9

Here's the breakdown: The first number stands for the number of carbons in the lipid. Since lipids are basically very large hydrocarbon chains with a universal amphipathic tail (either COOH or COO- when ionized), one element that differentiates them is the number of carbons.

The second number stands for the # of double bonds in the lipids. Double bonds indicate the saturation of the lipid. 0 means that there are no double bonds, and the lipid is saturated. 1 means the lipid is monounsaturated, there is one bend in the lipid which influences its chemical properties (and, incidentally, make it less unhealthy for you). Any number greater than one places the lipid in the polysaturated category.

The Δ9 represents the exact location of the double bond, the ninth carbon. Since the double bond is the location of the lipid's bend, its important to know whether this bend is in the middle, near the head, or near the tail: all of which also have a major influence on the lipid's chemical properties.