What is the difference between a CSTR and a PFR?

CSTR (Continuous Stirred Tank Reactor) has perfect mixing - composition everywhere equals outlet composition, concentration is uniform but low (at exit value). PFR (Plug Flow Reactor) has no axial mixing - fluid moves as plugs with concentration varying along length from inlet to outlet values. For positive-order reactions, PFR is smaller than CSTR for same conversion because average concentration is higher. CSTR is better for highly exothermic reactions (diluted conditions for temperature control).