As the triglyceride is effectively surrounded by hydrocarbons, it is hydrophobic (water-hating)
There are two types of fatty acids: saturated and unsaturated. Saturated fatty acids have no double bonds, and so the maximum amount of hydrogen molecules attached. Unsaturated has at least one carbon double bond, and so could potentially have more hydrogen. Monounsaturated fatty acids have just one carbon double bond, whereas polyunsaturated fats have 2 or more.
Triglycerides with unsaturated fatty acids (or with relatively long fatty acid chains) have a higher melting point, and so are solids at room temperature. These are known as fats, whereas saturated fatty acids (or with relatively short fatty acid chains) are known as oils and are liquids at room temperature. Plants generally produce oils such as olive or corn oil, and animals produce fats such as butter and lard, but there are exceptions to the rule.
The functions of triglycerides are multiple. The main use is as an energy source; they can be broken down and oxidised in respiration, and the energy produced used to make ATP. They can also be used as an energy store, as 1g of fat has twice the energy of 1g of starch or protein so more energy can be stored in a smaller space, so fat stores are lighter and smaller. As well as this, it is insoluble, so it doesn’t affect osmotic potential, so it doesn’t enter cell’s cytoplasm. It is stored in adipose tissue in animals, and plant seeds. The adipose tissue also works as an insulator in artic animals, as fat is a bad conductor of heat, and it has a low density so it is useful for aquatic life as a buoyancy aid. The adipose tissue is also quite spongy, so it helps to protect vital organs by absorbing shock. Another function of fats is as a water source, as when respiration occurs using fat + oxygen, water is produced as well as energy and carbon dioxide. This is why camels store large fat stores in their hump, so it can be broken down.
Phospholipids have a very similar structure to triglycerides, but instead of one of the fatty acid chains, it has a phosphate ion. This ion has a negative charge, which attracts water molecules to it, and so it is hydrophilic (water loving). This provides the phospholipid molecule with an odd characteristic, as one of its site id hydrophilic, and the other is hydrophobic. It is this property that is used to form a phospholipid bilayer in cell membranes. The hydrophilic head of the molecule is in the cytoplasm (which is made mostly of water with other substances dissolved in it) either inside or outside of the cell and the tails point away, towards the centre of the membrane. The tails are attracted to one another van der Waals forces and hydrophobic interactions that give the membranes strength. The hydrophobic property of the tails also means that the membrane doesn’t allow molecules to pass easily through them, which is why it has certain purpose built pores, which can allow passage to substances.
