Unsaturated fatty acids have one or more double bonds between carbon atoms. 

Ctwo carbon atoms in the chain that are bound next to either side of the double bond can occur in a cis or trans configuration.

 CIS
cis configuration means that the two hydrogen atoms adjacent to the double bond stick out on the same side of the chain. 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. For example, oleic acid, with one double bond, has a "kink" in it, whereas linoleic acid, with two double bonds, has a more pronounced bend. α-Linolenic acid, with three double bonds, favors a hooked shape. The effect of this is that, in restricted environments, such as when fatty acids are part of a phospholipid in a lipid bilayer, or triglycerides in lipid droplets, cis bonds limit the ability of fatty acids to be closely packed, and therefore can affect the melting temperature of the membrane or of the fat.

TRANS
A trans configuration, by contrast, means that the adjacent two hydrogen atoms lie on opposite sides of the chain. As a result, they do not 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 three n 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 (e.g., hydrogenation).
The differences in geometry between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in biological processes, and in the construction of biological structures (such as cell membranes).

Saturated fatty acid.

Saturated fatty acids have no double bonds. Thus, saturated fatty acids are saturated with hydrogen (since double bonds reduce the number of hydrogens on each carbon). Because saturated fatty acids have only single bonds, each carbon atom within the chain has 2 hydrogen atoms (except for the omega carbon at the end that has 3 hydrogens).

Numbering of the carbon atoms in a fatty acid

The position of the carbon atoms in a fatty acid can be indicated from the COOH- (or carboxy) end, or from the –CH3 (or methyl) end. If indicated from the -COOH end, then the C-1, C-2, C-3, ….(etc.) notation is used (blue numerals in the diagram on the right, where C-1 is the –COOH carbon). If the position is counted from the other, –CH3, end then the position is indicated by the ω-n notation (numerals in red, where ω-1 refers to the methyl carbon).

The positions of the double bonds in a fatty acid chain can, therefore, be indicated in two ways, using the C-n or the ω-n notation. Thus, in an 18 carbon fatty acid, a double bond between C-12 (or ω-7) and C-13 (or ω-6) is reported either as Δ12 if counted from the –COOH end (indicating only the “beginning” of the double bond), or as ω-6 (or omega-6) if counting from the –CH3 end. The “Δ” is the Greek letter “delta”, which translates into “D” ( for Double bond) in the Roman alphabet. Omega (ω) is the last letter in the Greek alphabet, and is therefore used to indicate the “last” carbon atom in the fatty acid chain. Since the ω-n notation is used almost exclusively to indicate the positions of the double bonds close to the –CH3 end in essential fatty acids, there is no necessity for an equivalent “Δ”-like notation - the use of the “ω-n” notation always refers to the position of a double bond.
Fatty acids with an odd number of carbon atoms are called odd-chain fatty acids, whereas the rest are even-chain fatty acids. The difference is relevant to gluconeogenesis.

Production
Industrial

Fatty acids are usually produced industrially by the hydrolysis of triglycerides, with the removal of glycerol (see oleochemicals). Phospholipids represent another source. Some fatty acids are produced synthetically by hydrocarboxylation of alkenes.

In animals, fatty acids are formed from carbohydrates predominantly in the liver, adipose tissue, and the mammary glands during lactation.