In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic tail (chain), which is either saturated or unsaturated.Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 12 to 28.Fatty acids are usually derived from triglycerides or phospholipids. When they are not attached to other molecules, they are known as free fatty acids.ost naturally occurring fatty acids have a chain of an even number of carbon atoms, from 12 to 28. Fatty acids are usually derived from triglycerides or phospholipids. When they are not attached to other molecules, they are known as free fatty acids.