The "little squares" are the NxN cubes inside the sudoku. For instance if you were given

123 456 789
234 567 891
345 678 912

456 789 123
567 891 234
678 912 345

789 123 456
891 234 567
987 654 654

All the lines and columns are correct. But none of the blocks is (the upper left block hold numbers 1,2,3,2,3,4,3,4,5).