Loop ... Into

LOOP AT itab INTO wa_tab.
  ...
  ...
  ...
  wa_tab-fld1 = ’X’.
  MODIFY itab FROM wa_tab.

ENDLOOP.

Loop ... Assigning

LOOP AT itab ASSIGNING .
  ...
  ...
  ...
-fld1 = ’X’.


ENDLOOP.

Loop ... Assigning is faster than Loop ... Into because when you modify field-symbol, system automatically update data in internal table.