Does it compare word by word or line by line?

Line by line, which is what makes a diff readable for prose and for code. A line that changed at all shows as one removal and one addition rather than as a scatter of character-level edits.

Yes, and it is a common use — comparing two config files or two versions of a function. Indentation is significant to the comparison, so a whitespace-only change will show up. That is usually what you want.

Is my text sent anywhere?

No. Both texts stay in the tab and the diff algorithm runs locally, which means you can safely compare production configuration or anything else you would not paste into a website.

Text Compare (Diff)

Paste two versions of a text side by side and every difference is highlighted — additions in green, removals in red, line by line. Useful for comparing two drafts, two config files or two pasted API responses. The diff is computed in your browser; neither version is uploaded.

Runs in your browser — nothing is uploaded

Questions

Does it compare word by word or line by line?: Line by line, which is what makes a diff readable for prose and for code. A line that changed at all shows as one removal and one addition rather than as a scatter of character-level edits.
Can I compare code?: Yes, and it is a common use — comparing two config files or two versions of a function. Indentation is significant to the comparison, so a whitespace-only change will show up. That is usually what you want.
Is my text sent anywhere?: No. Both texts stay in the tab and the diff algorithm runs locally, which means you can safely compare production configuration or anything else you would not paste into a website.