Organic molecules are those that contain carbon and hydrogen. In addition, they usually contain oxygen, nitrogen and sulfur atoms.
The atom, or the group of atoms, that characterizes a molecule's reactivity, or function, within a given family of organics is called a functional group.
It is the functional groups of amino acids that determine their behaviors in terms of chemical reactivity. By extension, it is the reactivity of the amino acid functional groups that often determine the shape and function of proteins.