Paste the original and the changed version and see exactly what was added and removed. Neither text leaves your browser.
Most online diff tools do the comparison on their server. You paste two texts, both are uploaded, the server works out the difference and sends the result back. For a shopping list that is fine. For a contract draft, a patient note, a legal filing, or unreleased code, it means the whole document now sits on someone else's machine, under their retention policy and their logs.
This page compares the two texts inside your browser using a small library that loads with the page from this site. There is no upload step because there is nowhere to upload to.
Words is the mode for prose. Edits to a sentence show up as the specific words that changed instead of the whole paragraph turning over.
Lines is the mode for lists, config files, and code, where a change usually means a whole line was rewritten, added, or dropped.
Characters is the finest grained. It catches a single-letter typo or a changed digit, and it is the noisiest of the three on longer text.
Moved text reads as a delete plus an insert. The comparison runs top to bottom, so a paragraph you moved from the start of the document to the end shows as removed in one place and added in another. Nothing tracks it as a move.
Word mode ignores changes that are only whitespace. If you need to see a double space, a stray tab, or a trailing space, switch to character mode.
Very large texts get slow. Comparing two documents of a few hundred thousand characters takes real work, and it all happens on your computer. The tool still runs it, and tells you when the wait is coming.
You don't have to take our word for it. Every browser has a built-in panel that shows everything a page sends or receives over the internet. On a computer (this doesn't work on a phone):