Paste two versions of a text to see exactly what changed: additions, deletions and what stayed the same, word-by-word or line-by-line.
Word mode compares word by word, so it highlights the specific words that changed inside a sentence. Line mode compares whole lines, which is closer to how code and structured documents are usually diffed.
Green (underlined in text form) marks text that was added in B and is not in A. Red with a strikethrough marks text that was removed from A and is not in B. Text with no highlight is unchanged.
Yes. "Ignore case" treats uppercase and lowercase as equal. "Ignore extra whitespace" collapses repeated spaces and blank lines before comparing, useful when only formatting changed.
It compares in your browser using a standard longest-common-subsequence algorithm, which is fast for a page or two of text. Very large inputs (many thousands of words) may be too slow in word mode; switch to line mode or compare a shorter section.
No. The comparison runs entirely in your browser. Nothing you paste is sent to a server or saved.