Paste the original and the revised version; the tool lines them up and highlights what was added, removed, or kept.
The comparison is line by line using a longest-common-subsequence match: lines found in both texts, in order, are "kept"; the rest are removals from the first text and additions in the second. It suits comparing drafts, configuration files, lists, and terms — anywhere whole lines change more often than single characters.
Why does a moved paragraph show as deleted and re-added? Line diffs do not track moves. A block that changed position appears as a removal at the old place and an addition at the new one — the content check still passes.
Is there a size limit? The matcher compares up to about 800 lines per side comfortably. Larger files will still run, just more slowly.