Once envisioned as a minor upgrade to Windows XP, Windows "Longhorn" took on all-new importance in early 2002 when Microsoft decided to reach for the brass ring and make this upcoming Windows release an all-encompassing major upgrade with a new security architecture called Palladium, a hardware 3D-enabled user interface, a brand-new, database-backed storage engine, and many more exciting new features. Here's the first--and most comprehensive--Longhorn FAQ ever created, constantly updated to include the latest information about this release.
Q: What is Windows "Longhorn"?
A: "Longhorn" is the codename for a major wave of technology and platform software from Microsoft. This generation of software will include new versions of Windows, Windows Server, .NET, MSN, Microsoft Office, and other products.
Windows "Longhorn" is the next major desktop Windows release, which will follow Windows XP; there is also a minor Windows Server revision that will ship in the Longhorn wave. Originally expected to be a fairly minor upgrade, Windows Longhorn will now include a number of new features including a revised task-based (or "inductive") user interface, an extensible, dock-like, Sidebar, and a SQL Server 2003-based storage engine called WinFS (Windows Future Storage). Microsoft said that Longhorn would be a desktop-only release in November, 2002, when the company told me that, "Customer requirements dictate our release strategies and timing for Windows products.
News source: Paul Thurrott's SuperSite