The difference is more philosophical than physical. In second quantization one starts from particles as basic primitive objects and then develops mathematical tricks for efficient description of many-particle states obeying either Bose-Einstein or Fermi-Dirac statistics. These mathematical tricks can be written in terms of abstract mathematical objects called quantum fields. In QFT one starts from quantum fields as basic primitive objects and then derives that certain quantum states of those fields can be interpreted as particles obeying either Bose-Einstein or Fermi-Dirac statistics. In short,Please teach me the difference between second quantization and QFT?