The majority of proteins contain secondary and tertiary structure and some proteins also contain a fourth and final level of structure known as quaternary structure. So a protein is said to have quarter structure if that protein actually consists of two or more individual polypeptide chains and the quarterliary structure is basically the interaction of these polypeptide chains with respect to one another. Now, the simplest type of quaternary structure is a dimer. In a dimer we have two individual polypeptide chains and these polypeptide chains can interact usually via non covalent bonds but sometimes we have covalent bonds such as disulfide bridges disulfide bonds that also hold those individual polypeptide together. Now, generally speaking, whenever we have quotinary structure those individual polypeptide chains are also known as subunits. So for example, in a dimer we have two subunits.