There are two methods to purify polyclonal antiserum or monoclonal ascites tissue culture supernatant. Protein A/G (Protein A or protein G) purification is mainly to utilize the high affinity of Staphylococcus aureus protein A or Staphylococcus protein G to the immunoglobulin Fc domain. Affinity purification makes use of reversible interactions between the proteins and a specific ligand coupled to a chromatographic matrix.
In simple terms, the affinity purification isolates peptide-specific antibodies, while protein A/G (Protein A or protein G) purification purifies total immunoglobin (Ig) from the serum. So the protein A/G purified antibody contains both antigen-specific antibody and endogenous Ig of the host which may not be specific to the antigen.
Usually for ELISA, WB applications, protein A/G purification shall be fine. For some other applications requiring high specificity, e.g., IHC, IP, etc, affinity purification is recommended.