Mainly the number of populated fields at the start and the tactics needed to solve the puzzle determine the difficulty level of a sudoku. It’s always hard to have a real standard for difficulty levels so they might differ between sudoku providers and there is a chance there even is some difference within a level by a specific provider. In the end, it’s also personal. Some people prefer a certain tactic or think in a certain way.
Love Sudoku offers sudoku’s in 4 difficulty levels: easy, medium, hard and expert. Our advice is always to start low and move up to a level that challenges you, but in a comfortable way.