Found a sudoku in a magazine? Paste it in to see how hard it really is. The rater gives you a quick estimate so you know what you're getting into.
The rater looks at two things: how many clues the puzzle gives you, and how much our solver has to "think" to crack it. More thinking means more branching paths, which usually means a harder puzzle for a human.
It's a quick estimate, not a perfect rating. Two puzzles labelled "hard" can feel quite different depending on which technique unlocks them. Use it as a guide.
Difficulty depends partly on which techniques you know. A puzzle that needs X-wing might be rated hard, but if you've never seen X-wing it'll feel impossible.
Roughly. Different publishers use different scales. Our easy is usually their easy or beginner; our evil matches their diabolical or nightmare.
Yes. The rater doesn't care where the puzzle came from.