Sudoku is played on a 9×9 grid divided into nine 3×3 boxes. Some squares start filled in. Your job is to fill the rest so that every row, every column and every box contains each number from 1 to 9 exactly once. A well-made puzzle has one solution and never needs guessing.
Getting started: pick a number, say 1, and look at each box that doesn't have it yet. Rows and columns that already contain a 1 rule out squares in that box. When only one square is left, write the 1 there. That's a hidden single, and it will take you a long way.
Next, try the other direction: pick an empty square and check which numbers its row, column and box already use. If only one number is missing, that's a naked single. Using pencil marks (small notes of possible numbers) makes this easy, and PuzzleCraft clears notes automatically as you place numbers.
As puzzles get harder you'll need techniques that remove candidates instead of placing numbers: naked and hidden pairs, pointing pairs, triples, box/line reduction and finally patterns like the X-Wing. Each one has its own short lesson in the Learn Sudoku section.
Stuck? The Hint Assistant teaches instead of just giving answers. The first hint points to the area to look at, the second names the technique, the third highlights the exact squares and explains the logic in plain English, and only the fourth fills in the number. Hints used appear on your results card.