Help regarding Sudoku Online
Next to each Sudoku there is number between 100 and a few ten thousand, which is a difficulty rating.
If this number is high, the Sudoku is usually harder to solve.
This rating is calculated as follows: A program solves the Sudoku using a fairly simple solving technique and backtracking.
For each step with the simple method one point is added to the score, for each backtracking step 7 points are added.
To get a feeling what a certain difficulty rating means, you can peek into the Statistics.