Cl2 (g) + 4 Na2S2O3 (aq) → 2 NaCl (aq) + 5 S (s) + 3 Na2SO4 (aq)
This is an oxidation-reduction (redox) reaction:
5 SII + 10 e- → 5 S0 (reduction)
3 SII - 12 e- → 3 SVI (oxidation)
2 Cl0 + 2 e- → 2 Cl-I (reduction)
Na2S2O3 is an oxidizing agent, Na2S2O3 is a reducing agent, Cl2 is an oxidizing agent.