Protein domains are portions of the protein that are structurally stable by themselves.... that is, without the rest of the protein. They are conserved through evolution because they perform an important task.
Proteins can be one polypeptide chain (monomeric), two polypeptide chains (dimeric), etc.
A domain can be within a single peptide chain (subunit) or it can encompass more than one subunit, so do not confuse subunit with domain, since domain is based on preservation and function, and "stand alone" stability.
Different subunits can be associated through different interactions, such as electrostatic, hydrophobic, disulfide bridges, etc.