This puzzle goes by the name "15-puzzle" when played with four rows and four columns. You can pick from 29 different varieties based on the number of rows and columns.
Slide the numbers around until they are in numerical order from least to greatest, left to right and top to bottom.
The white space is the open space that the numbered squares are moved to. To move a number, click on it. Only
numbers that are immediately to the left, right, above or below the empty space can be moved. The open
space should end up in the lower right hand corner when the puzzle has been solved.
You choose a puzzle configuration (rows and columns) from the dropdown at the top. You can select the checkbox "Just Squares" to only show square puzzles in this dropdown.
There is a reset button (pair of blue arrows) that can be used to reset the puzzle back to its original state. This also resets the number of moves to 0 and resets the timer so you can try again.
There is also a new puzzle button that changes the starting arrangement. The timer starts when you make your first move.