Lipids are primarily found in membranes. This includes both the plasma membrane and membranes surrounding particular organelles. Lipids are very useful in membranes because their nonpolar nature helps them act as a barrier between the cell and the outside environment.

Remember that steroids are derived from cholesterol, a four-ringed structure. The four-ringed structure is a constant in all cholesterol-derived molecules. Variation between molecules comes from the final constituent on the four rings.

Plasma membranes are mostly made up of phospholipids. Phospholipids are lipid molecules that contain both polar and nonpolar regions. This amphipathic nature of phospholipids is very important in plasma membranes. Several hormones of the endocrine system are derived from steroids (a type of lipid). Some examples of steroid derived hormones include testosterone, estrogen, and cortisol; therefore, lipids are precursors to molecules in the endocrine system. By definition, unsaturated fatty acids contain carbon-carbon double bonds.

Glycerol is a three-carbon molecule and forms the backbone of triglycerides and phospholipids. Triglycerides have three fatty acids, one attached to each of the carbons in glycerol. Phospholipids use two glycerol carbons to bind fatty acids, and the third to bind a phosphate group.

There are several types of lipids, including fatty acids, phospholipids, sphingolipids, and sterol lipids as well as many more.

Triglycerides fall under the category of glycerolipids and contain a three-carbon glycerol backbone with three fatty acids tails. Phospholipids contain a polar phosphate group, a glycerol backbone, and two fatty acid tails. Sterol lipids contain a four-ring base structure. Cholesterol and steroids are sterol lipids.

Glucose is a monosaccharide (carbohydrate), not a lipid.

Lipids are the fat molecules of biology and are frequently composed of fatty acids. Glycerol is a short 3-carbon chain that is often used to bind different fatty acids together and form lipid structures. When glycerol binds to three fatty acid chains, it forms a triglyceride. Triglycerides are used to store energy in the body.

Phospholipids, which make up the cell membrane, are formed when a glycerol binds to two fatty acids and a phosphate group. Phosphates are complex ions formed from one phosphorus atom and four oxygen atoms: . Steroids are another type of lipid, but do not contain glycerol.

Carbohydrates, proteins, and nucleic acids are the other primary classes of biological macromolecules, and generally do not contain glycerol. Nucleotides are the monomers of nucleic acids and are composed of a pentose sugar, a phosphate group, and a nitrogenous base. Carbohydrates are formed from monosaccharide monomers.

Hydrophilic means "water loving." Hydrophobic means "water fearing." Recall from your general chemistry, "like dissolves like" with regards to solubility. This means that hydrophobic molecules can be dissolved in hydrophobic solvents, and vice versa. Lipids such as oil and cholesterol are hydrophobic molecules and are thus insoluble in water. A good rule of thumb is that all acids are soluble in water. We know sugar is soluble in water, and starch has a similar molecular structure as sugar; it consists of many sugar molecules in a chain.

A saturated fat contains no double bonds in its fatty acid chain. Just remember that saturated means the fat is saturated with hydrogens. Double bonds eliminate two hydrogen atoms per occurrence, and are present in unsaturated fats.

Phospholipids contain two fatty acid chains bound to a hydrophilic head (usually glycerol). Fats (triglycerides) contain three.  

Body fat, also known as triglycerides or triacylglycerols have three fatty acid chains. These hydrocarbon tails are hydrophobic, and they are each attached to the glycerol head, which is hydrophilic via ester linkages. Adipose tissue is a type of connective tissue that plays a structural and protective role, provides stored energy, and helps regulate body temperature.  

A fat is a lipid with three fatty chains linked by an ester linkage to glycerol.