What "diff" means
A diff is a description of the changes between two versions of a text. The name comes from the Unix diff command, first released in 1974. It answers one question: given two texts, what lines (or words) were added, removed, or left unchanged?
Most diff tools use the Myers algorithm, which finds the shortest edit script — the minimum number of insertions and deletions needed to transform one text into the other. The result is presented as a set of hunks: grouped changes with surrounding context so you can see where in the file each edit sits. Word-level diff goes further, comparing within lines to highlight exactly which word changed rather than flagging the entire line.
When to use a text diff tool
The most common use case is comparing two versions of a config file when you're not sure what changed between deployments. Paste the old version on the left, the new version on the right, and the diff highlights every modified line instantly. This is faster than reading both files manually and more reliable than memory.
Text diff is also useful for reviewing pasted API responses. If an API returns different data on two calls and you're debugging the discrepancy, a diff shows exactly which fields changed without requiring you to scan through JSON manually. Other common uses: checking whether a copied document picked up unwanted edits, verifying that a translation preserved the same paragraph structure as the source, and auditing a vendor-supplied config against your internal baseline.
For structured data with a known schema, a dedicated tool gives better results. Use JSON diff checker when comparing JSON — it understands the structure and highlights field-level changes rather than treating the whole document as flat text.
How to compare two texts
Open the text diff checker. Paste your original text into the left panel and the modified version into the right panel. The diff runs automatically as you type or paste — no button to click. Lines with additions appear highlighted in green. Lines with deletions appear in red, sometimes with strikethrough. Lines that didn't change appear as plain context.
If you're comparing long documents, use the line-jump controls to skip between change hunks rather than scrolling through unchanged sections. For documents where individual word changes matter more than line changes — a paragraph rewritten in place, for example — switch the tool to word-level mode. This compares within each line and marks the exact words that changed, not the whole line.
Reading the diff output
Green highlighting marks text that exists in the new version but not the old — additions. Red marking (or strikethrough) marks text that existed in the old version but was removed. Unchanged lines appear in both panels without any highlight — they are context, helping you locate where the change sits in the overall document.
A line that was modified typically shows as a paired deletion and addition: the old line in red on the left, the new line in green on the right. At word level, you may see a single line where only a few words are highlighted, making it clear that the sentence structure stayed intact but specific values changed. If every line is red on the left and green on the right, the two texts share no common lines — this usually means you've pasted unrelated content, or the whitespace and line endings differ (Windows CRLF vs Unix LF).
Diff tools vs version control
Git's built-in diff is the right tool for tracked files in a repository. It knows the file history, can diff between any two commits, and integrates with pull request reviews. A standalone text diff tool fills a different gap: files you don't have in version control, content copied from external sources, or situations where you simply need a quick comparison without a git working tree.
Common examples: you receive two versions of a configuration file from a client over email, neither in a repo. Or you copy a snippet from a wiki and want to confirm it matches the version from last month. Or you're testing a regex replacement and want to verify the output against the input. In all these cases, pasting into a diff tool is faster than initializing a repo, committing both versions, and running git diff.
Finding what changed
A diff tool's value is speed and precision. Instead of reading two texts in parallel and tracking differences mentally, you get a highlighted, line-by-line comparison in under a second. For routine comparisons — configs, API responses, copied documents — it removes the error-prone manual process entirely.
The text diff checker runs in the browser with no upload, no account, and no size limit imposed by a server round-trip. Paste both texts and the result is immediate. For JSON specifically, the JSON diff checker gives field-level visibility that flat text diff can't provide.