A diff (short for "difference") compares two pieces of text line by line and shows what changed between them. It's a fundamental tool in software development (used by Git, code reviews, and version control systems), but it's also highly useful for comparing contracts, essays, configuration files, or any two versions of a document. This tool uses a longest-common-subsequence (LCS) algorithm to produce an accurate, line-by-line comparison instantly in your browser.