Compare two pieces of text and see exactly what changed
Comparing two versions of the same text by eye gets unreliable fast — a paragraph reworded, a line moved, a single character changed in a long password or a code snippet are all easy to miss on the page but obvious once the two versions are lined up against each other and the differences are marked. This is the same basic idea behind the "diff" shown by version control systems, word processors' track-changes view, and code review tools, just applied to whatever text is pasted in rather than to files already known to a repository.
Finding the difference between two sequences is a genuine algorithmic problem — the goal is the smallest possible set of additions and removals that turns one sequence into the other, not just any set that happens to work, since a naive comparison can flag far more as "changed" than actually moved. Line mode treats each line as one unit and is the natural choice for documents, code, or lists; word mode breaks both sides into words so a change inside a single sentence doesn't mark the whole line as different; character mode goes finer still, useful for spotting a single-letter typo or a one-digit change buried in a long value.
Ignoring case or whitespace differences filters out changes that usually don't matter — trailing spaces, re-indentation, or capitalization — so the highlighted result reflects the changes that are actually meaningful for the comparison at hand. Both pieces of text are compared entirely on this page; nothing is uploaded anywhere.