Paste the old and the new text and see only what changed. Unchanged lines are hidden. Your texts never leave this computer.
The tool compares the two texts line by line and finds the longest sequence of lines that appears in both in the same order. Lines outside that sequence are marked as added or removed. Most version control tools use a similar idea.
On this page the "Show only differences" option is on: a few lines around each change are visible and the other unchanged lines are hidden. Turn the option off to see everything.
Yes. With the matching options ticked, lines that differ only by letter case or by leading, trailing or extra spaces are not counted as changed.
No. The comparison runs in your browser and the texts are neither uploaded nor stored.
It works smoothly up to a few thousand differing lines. For much larger texts a warning is shown to protect the browser, and you can split the text into parts.