A User-Agent string is the short, semi-structured text every browser sends with each request, packed with the browser name and version, rendering engine, OS, and device type, but it's dense and inconsistently formatted across vendors. This tool runs it through the UAParser.js library to break it into labeled fields you can actually read, whether you're debugging analytics data or checking your own browser's string.
It's historical, early browsers spoofed Netscape's UA string to pass compatibility checks on old sites, and the "Mozilla/5.0" prefix stuck around as a de facto standard even in modern Chrome, Safari, and Edge.
Yes, it's just a header the client sends, so a browser extension, bot, or developer-tools device emulation mode can set it to any value it wants. Treat parsed results as reported, not verified.
Chrome forked Blink from WebKit back in 2013; the UA string still mentions "WebKit" for legacy compatibility even though Blink is the engine actually rendering the page today.