Classic Outlook (Windows): a ribbon interface with File, Home, Send/Receive tabs across the top — the long-standing desktop version.
New Outlook: a simplified, flatter interface with a toggle switch labeled "New Outlook" in the top-right corner — Microsoft's newer version, gradually replacing classic Outlook by default.
Outlook on the web: whatever you see when you sign in at outlook.com or office.com in a browser, rather than a separate installed application.
Outlook for Mac: the standard Mac desktop version, distinct from both New Outlook and web Outlook.
