On function for Lipids is that of Energy storage. Lipids contain a lot of calories in a small space. Since Lipids are generally insoluble in polar substances such as water, they are stored in special ways in you body's cells. Lipids can also function as structural components in the cell. Phospholipids are the major building blocks of cell membranes. Lipids are also used as hormones that play roles in regulating our Physiology (metabolism). Most lipids are composed of some sort of fatty acid arrangement. The fatty acids are composed of methylene (or Methyl) groups, and are not water soluble.
Fatty Acids: The lipid building blocks: The common building block for most of the different types of lipids is the fatty acid. Fatty acids are composed of a chain of methylene groups with a Carboxyl functional group at one end.
The methyl chain is the fatty part, the Carboxyl, the acid. The fatty acid chains are usually between 10 and 20 Carbon atoms long. The fatty "tail" is non-polar (Hydrophobic) while the Carboxyl "head" is a little polar (Hydrophillic).
Fatty acids can be saturated (meaning they have as many hydrogens bonded to their carbons as possible) or unsaturated (with one or more double bonds connecting their carbons, hence fewer hydrogens). A fat is a solid at room temperature, while an oil is a liquid under the same conditions. The fatty acids in oils are mostly unsaturated, while those in fats are mostly saturated.
Compare the fatty acid on the left to the one above. A double bond connects the two, red Carbon atoms. Since Carbon forms four bonds, these Carbon atoms are only bonded to one Hydrogen each. This is not as many as the fatty acid above, so this fatty acid is called unsaturated. (monounsaturated).
The double bond also gives unsaturated fatty acids a bend in the methylene chain. This bend affects the chemical characteristics of unsaturated fatty acids. The straighter, saturated Fatty Acids all line-up very close together and stick to each other. These interactions make them less fluid and more solid (more like Fat). The bent unsaturated Fatty Acids can't get as close together, so they don't stick as much. They are more fluid (more like Oil).
Triglycerides: Energy Storage, Three fatty acids bonded to Glycerol. Triglycerides are Energy-storage molecules. They are formed by connecting three fatty acids (shown in black) to the red part of the molecule on the left, Glycerol. As you can imagine, the three fatty acids together, contain a lot of Energy (aka Calories). Fat has a lot of calories.
The flabby stuff most of us have on certain parts of our bodies is cells filled with triglycerides. In trigylcerides, a fatty acid is joined to each of the three Carbons of Glycerol by Dehydration Synthesis to form a molecule which stores a lot of calories in a small space.