Fat is our secondary energy source, the first being carbohydrates. Fats are our most concentrated form of energy - we derive 9 food calories (kcals) from a gram of fat, compared to 4 kcals per gram of protein or carbohydrates.
Fat acts as an energy reserve and is stored as fatty tissue (located subcutaneously & surrounding organs). An 'ideal' bodyfat percentage depends on your age and exercise regimen and there are obvious related health implications should you have too much, or too little, fat storage.
Saturated and unsaturated are the two fat classification within our foods. Foods high in saturated fat often recognised as being the 'unhealthy' option. See our 'Tell me more about' bar for related subjects, including "healthy fats" like those found in Salmon (pictured).