Fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually not found in organisms, but instead as three main classes of esters: triglycerides, phospholipids, and cholesterol esters. In any of these forms, fatty acids are both important dietary sources of fuel for animals and they are important structural components for cells.
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A fat is a type of lipid which comes in the form of many different molecules throughout the body, and fatty acids are just one type of fat that can be found. 
Fatty acids come in a variety of molecular shapes, sizes and chemical compositions. They can also be divided into many different categories. Fatty acids can become attached to different molecules like carbohydrates or proteins and can elicit a wide range of functions in the body. Some of those functions include: providing long-term energy storage, insulation (prevents heat loss, protects vital organs, helps transmit nerve impulses faster), structure, transportation around the body for nutrients and other biological molecules, and can also be precursors to other compounds in the body such as hormones (some of which are important for gut/immune/overall health). The length, degree of saturation and configuration of a fatty acid affects how it is broken down, absorbed and utilized in the gastrointestinal tract (GIT).