Text File Generator
Two versions of the same document, and no memory of what changed between them. That is the problem this text diff checker solves. Paste the old version into the left box and the new one into the right, and every line that was added shows up marked with a plus, every line that was removed gets a minus, and everything untouched sits between them in plain grey.
Reading the comparison
The count at the top tells you the size of the change before you read a single line. Three added and three removed usually means three lines were edited rather than six lines being touched, since a modified line shows up as its old version removed and its new version added.
When the two texts are long and mostly identical, turn on the changed lines only option. Everything unchanged disappears and you are left with a short list of exactly what moved, which is far easier to scan than scrolling through four hundred grey lines hunting for colour.
When formatting changed but the words did not
Copying text between apps has a habit of introducing double spaces, trailing whitespace, and capitalisation changes that a strict comparison reports as differences. The ignore extra spaces option collapses runs of whitespace and trims the ends of each line before comparing, and ignore case does the obvious thing, so you can see whether anything of substance actually changed underneath the noise.
Both options only affect the comparison, never the text itself. What you see in the result is your original lines exactly as you pasted them, just judged more loosely.
It compares lines, not individual words
This is worth knowing before you use it on prose. If you change one word in a long paragraph, and that paragraph is a single line, the whole paragraph is reported as removed and re-added. The change is found correctly, but you still have to read the two versions to spot which word moved.
The fix is to work with text that is already broken into shortish lines, which is why this suits code, configuration files, lists, and CSV data so well. For a wall of prose, breaking it into one sentence per line before pasting turns a useless block comparison into a precise one.