From NuGOwiki

Fatty acids are straight chain hydrocarbons possessing a carboxyl (COOH) group at one end. An essential fatty acid cannot be synthesized de novo by the organism, and therefore must be supplied in the diet. Omega-3 fatty acids are polyunsaturated fatty acids which have a C=C double bond in the ω-3 position. The carbon next to the carboxylate is known as α, the next carbon β, and so forth. Since biological fatty acids can be of different lengths, the last position is labelled ω, the last greek letter, even though is is almost never in the 23rd position. Since the physiological properties of unsaturated fatty acids largely depend on the position of the first unsaturation relative to the end position and not the carboxylate, the position is signified by (ω minus n). For example, the term ω-3 signifies that the first double bond exists as the third carbon-carbon bond from the terminal CH3 end (ω) of the carbon chain. The number of carbons and the number of double bonds is also listed; ω-3 18:4 (stearidonic acid) indicates an 18-carbon chain with 4 double bonds, and with the first double bond in the third position from the CH3 end. The human body cannot synthesize omega-3 fatty acids from scratch, but can synthesize all the other necessary omega-3 fatty acids from the omega-3 fatty acid Alpha-linolenic acid. See also the Wikipedia link on omega-3 fatty acids.