How the comparison works
Both texts are split into lines and compared to find the longest sequence of lines they share in common - everything outside that shared sequence is marked as either removed (only in the original) or added (only in the changed version). This line-based approach is the same core idea used by most code diff tools and version control systems, which makes it easy to spot exactly what changed between two drafts, versions of a contract, or pieces of content.
Why line-level comparison, not word-level
Comparing whole lines at a time suits content that's naturally organized into discrete lines - code, configuration files, structured lists, or any text where each line represents one complete unit. For prose or essay-style writing where you want to see exactly which individual words changed within an otherwise identical sentence, the companion word-level Diff Checker tool on this site is the better fit.
Reading the added/removed summary
The summary line at the top gives a quick count of how many lines were added and removed in total, before you scan the detailed output below - useful for getting a sense of how substantial the changes are between two versions before digging into the specifics of what actually changed.
A limit worth knowing: a single changed word flags a whole line
Because comparison happens at the line level, changing even one word within an otherwise identical line causes that entire line to show as removed from the original and added in the revision, rather than highlighting just the specific word that changed - this is expected behavior for a line-based diff, and it's exactly why the word-level Diff Checker exists as a complementary tool for that more granular view.