Windows XP was the direct sequel to Windows 2000, officially released in the U.S.A on August 24, 2001, and in Japan on September 6th, 2001. It was released for all x86 compatible platforms.
Windows XP marked a drastic change from the original Windows formula, going for what many fans considered to be a "Fisher Price look". Windows XP also introduced built in Digital Restrictions Management in the form of activation. Because of all of this, XP wasn't very popular early on. But a combination of deceptive marketing and "improvements" via Service Packs caused it to somehow be called the "best OS ever" by kids who don't even know how to configure Windows 95 properly.
Windows XP would be directly followed up by Windows Vista around 2006, but a spin-off named Windows Server 2003 would create a divide between server and workstation versions of Windows for years to come.