Somebody asked me if I were to talk about the relationship between Lean, Scrum and XP what would that be in the "simplest way possible" (At our organization, we are implementing all three). Here's how I answered:
Lean introduces the principles, or the "why's" of producing software. Focus is on maximizing customer value while minimizing waste to provide the most optimal ROI.
Scrum takes those principles and introduces the management practices, or the "when's" of producing software. Focus is on incremental delivery of working solutions that improve over time.
XP takes Lean principles and Scrum practices and introduces the technical skills, or the "how's" of producing software. Focus is on constant improvement of skills (and the underlying infrastructure) through analysis, design, and implementation of producing software.
It is my strong feeling that you really need all three to maximize your effectiveness in the development of software. Individually, each part can be implemented on it's own but you will discover something missing. Together, these really cover all aspects of developing a successful software solution.