Text Compare highlights the difference between two versions of a text: additions in green, removals struck through in red, so you can see exactly what changed without reading both versions side by side yourself.
Words mode is best for prose, like comparing two drafts of a paragraph. Lines mode treats each line as a unit, which is usually clearer for code, config files, or lists where whole lines get added, removed, or reordered.
Very large inputs are capped, since the comparison algorithm's memory use grows with the product of both texts' lengths. If you hit the limit, try Lines mode (fewer, larger tokens) or compare smaller sections.
No. The comparison runs entirely in your browser. Nothing you paste is sent to a server.