Fatty acids are made up of a hydrocarbon with a carboxylic acid group on one end. The ones we are working with contain a straight chain hydrocarbon (no branching). In order to keep track of where things are on the molecule, the carbon atoms are assigned a number. Since the most interesting part of the molecule is the acid group, that carbon atom is assigned the number 1. If the molecule has 18 carbon atoms - the carbon atom on the other end of the line will be assigned the number 18.

One way to group fatty acids is by there degree of "saturation" - saturated, mono-unsaturated, and poly-unsaturated. The three fatty acids below illustrate the three groups. The chemical structures of these fatty acids can be found at: http://www-medlib.med.utah.edu/NetBiochem/fa_new/3_3.html

Saturated
Common name: stearic acid
Shorthand: 18:0
  The 18 indicates that there are 18 carbon atoms in the fatty acid.
  The 0 indicates that there are no double bonds between the carbon atoms, so this is a saturated fatty acid.

Mono-unsaturated
Common name: oleic acid
Shorthand: 18:1 cis-9
  The 18 indicates that there are 18 carbon atoms in the fatty acid.
  The 1 indicates that there is one double bonds between the carbon atoms, so this is a mono-unsaturated fatty acid. To complete the shorthand, we need to indicate the location of the double bond (by indicationg the carbon atom number where the bond starts) and the orientaion of the molecule at that bond (cis or trans). Oleic acid has the double bond located at the number 9 carbon and the orientation of the bond is cis.

Poly-unsaturated
Common name: linoleic acid
Shorthand: 18:2 cis-9 cis-12
  The 18 indicates that there are 18 carbon atoms in the fatty acid.
  The 2 indicates that there are two double bonds between the carbon atoms, so this is a poly-unsaturated fatty acid. Linoleic acid is an Omega-6 fatty acid. Counting from the tail of the fatty acid (the end of the molecule opposite the acid group) the first double bond encountered is on the sixth carbon atom. To determine the Omega series from the shorthand notation, subtract the position number of the highest number double bond (here it is 12) from the total number of double bonds (here it is 18) which for linoleic yields 6 so it is an Omega-6 series fatty acid.