Drop two .xlsx, .csv or .tsv files and open the Data tab to compare them as tables. Rows are matched by the first column when it holds a unique key, so sorting or reordering rows doesn't show up as a change. Each changed cell shows its old and new value.
Columns are matched by their header, and any column that was added or removed is listed separately. Excel workbooks are converted to CSV in your browser; nothing is uploaded.
By the first column when its values are unique in both files, such as an id or SKU. Otherwise rows are matched by their order.
Each sheet is included with a heading. The Data view works best with one sheet per file, so export the sheet you care about as CSV if needed.
Not to compare it. The comparison runs entirely in your browser, and the text you paste and the files you open are never sent to a server for that. There are no ads. The site counts visits with Google Analytics, which sees which page you opened, and keeps its own anonymous tally of what kind of comparisons are made (for example “a Word file, about 500 lines”), but neither ever sees the text you compare. If you copy a share link, the text is packed into the link itself, in the part after the # that is never sent to a server or to analytics. Your history and saves stay in your browser unless you sign in. If you do, they are uploaded to your account, but only after they are encrypted in your browser with a passphrase only you know, so nobody else can read them. The site stores your email address and that encrypted data, and can see how much you have saved but not what it says.