A. Check this Wikipedia Page!
Q. Where can I see the source code?A. For C++, check this! For this webpage, check this!
Q. Why does it sometimes fail or take too long time to find a solution?
A. First, it could be because the board size is too small to find the solution. Try a wider board.
Second, it's because the board was designed to use a randomized start column for the top-row queen to show the various possibility of the solution.
Try to "Solve" it again. Also, please remember it could take lots of time to solve a too-wide board.