Most fat stored in the human body is subcutaneous (which means "under the skin"). This layer of fat is made up of fat cells. A thin person still has between 20 and 27 million fat cells, while an overweight person can have anywhere from 75 to 300 billion fat cells distributed around the body.
Men and women store fat in different places on the body. Most females store subcutaneous fat in the breasts, waist, hips, and buttocks. Most males store fat in their chest, abdomen, and buttocks. This explains why a woman becomes "hippy" while a man develops a gut.
Your body also stores fat around the kidneys, the liver, and even inside muscle. Actually, as people age, fat storage within muscle increases. If a person becomes more inactive, muscles shrink and fat replaces them.
Your body stores fat today to use as energy tomorrow. When your body senses that a certain muscle requires more power, it sends enzymes to a fat cell to break down the contents of it, which releases glycerol and fatty acids into the blood. As the circulating fatty acids reach muscle that needs the extra energy, the muscles hold onto them and convert them into the required boost of energy.