A fatty acid consists of a chain of hydrocarbons with carboxylic acid at one of it's end.Due to the presence of the carboxylic acid group ( ), it is called a fatty acid.Now, fatty acids are categorized into two types- saturated and unsaturated.If the long carbon chain consists of double bond it is known as unsaturated fatty acid and if it does not contain double bond then it is known as saturated fatty acid. If double bonds are present, they can be reduced by the introduction of hydrogens into those bonds but once they are reduced and no double bonds are left, the chain is filled completely and there is no scope for the introduction of any further hydrogens and so this is completely saturated with hydrogens and thus known as a saturated structure. Similarly, as the double bonds can take up more hydrogens they are unsaturated.