Triglycerides.
A triglyceride is the most common form of lipid. A lipid can be oils, fats or waxes. A triglyceride is composed of one glycerol molecule and three fatty acid chains. The fatty acid molecules have long tails which are a composition of hydrocarbons (carbon chains with branches of hydrogen atoms). The fatty acid tails in triglycerides are hydrophobic (or water hating - they repel water). Their water repelling property makes them insoluble. The fatty acid tails always consist of the same basic structure. They have a carbon atom with one hydrogen atom branching off it an a double bonded oxygen atom also branching off of it. The carbon also is attached to the R tail. The R is the only exception of difference. The hydrocarbon tail is represented with an R.