The association of several protein chains into a larger structure, as in the case of hemoglobin is an example of a complex structure.
Biological structures are mostly organized hierarchically from simple structures to complex structures. The simplest structures are the monomer units of macromolecules. In protein, the monomer units are amino acids which when linked together in a linear formation make up the primary structure of protein.
primary protein polymers go on to form secondary, tertiary and quaternary structures, each level more complex than the previous.