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Polymers are classified along four independent axes — master all four for JEE. (1) By source: Natural (proteins, cellulose, rubber), Semi-synthetic (cellulose acetate, vulcanized rubber), Synthetic (PE, nylon, Bakelite). (2) By structure: Linear (HDPE, nylon — chains packed closely), Branched (LDPE — irregular packing), Cross-linked (Bakelite, vulcanized rubber — 3D network). (3) By polymerization: Addition/chain-growth (C=C monomers, no by-product: PE, PVC, PS, Teflon, PAN) and Condensation/step-growth (bifunctional monomers, lose H2O/HCl: nylon, polyester, Bakelite). (4) By molecular forces: Elastomers (weak forces + slight cross-linking: rubber, Buna-S), Fibers (strong H-bonding: nylon, Terylene), Plastics (intermediate: PE, PS). Additionally: Thermoplastics (remoldable: PE, nylon) vs Thermosetting (permanent 3D network: Bakelite, melamine). Homopolymer (one monomer: PE, PVC) vs Copolymer (two+ monomers: Buna-S, nylon-6,6). Key distinction test: Addition polymer has same empirical formula as monomer; condensation polymer's repeating unit differs from monomers (atoms lost). JEE questions typically ask: classify a given polymer, identify monomers from structure, or predict properties from structure. The four axes are independent — a polymer is classified on ALL four simultaneously (e.g., nylon-6,6 = synthetic, linear, condensation, fiber).