Project Based Learning is any learning environment in which the problem drives the learning. 

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The problem is posed so that the students discover that they need to learn some new knowledge before they can solve the problem.

Posing the problem before learning tends to motivate students.  They know why they are learning the new knowledge.

Problems are relevant, contextual and ill-structured.  It is in the process of struggling with actual problems that students learn content and critical thinking skills.