This is an example of a completed Frame Sudoku Puzzle.

The usual rules of Sudoku apply, but the grid is also surrounded by 'Frame' clues. These clues are the total of the first 3 cells in that given row/column

The first clue on the left-side of the grid is 14, and you can verify that the first 3 cells do add up to 14: 3 + 7 + 4 = 14

The first clue at the bottom of the grid is 24, and you can verify that 8 + 7 + 9 = 24.

Additionally, there is an extra 'trick' that you need to know for Frame Sudoku that you will need to use for the harder difficulties.

We can work out that each row and column must add up to 45 (1+2+3+4+5+6+7+8+9 = 45).
We know that the left-most 3 cells in the top row add to 18, and the right-most 3 cells add to 16.
This means that center 3 (highlighted) cells must add to 45 - 18 - 16 = 11

You will need to use this 'trick' to solve the harder puzzles.