"The best way for students to learn science is to experience problems that challenge science, and the thought, habits of mind and actions associated with trying to solve them. This implies opportunities for authentic, inquiry-based learning. Problem-based learning (PBL) is a powerful vehicle for this, in which a real-world problem becomes a context for students to investigate, in depth, what they need to know and want to know (Checkly, 1997). It is a robust, constructivist process, shaped and directed primarily by the student, with the instructor as metacognitive coach. 

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PBL is not just another iteration of what many science educators already use in their classrooms. To be truly "problem-based", Gallagher (1995) emphasizes, all threeof these key features must be present: initiating learning with a problem, exclusive use of ill-defined problems and teacher as metacognitive coach."

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http://www.cct.umb.edu/pblscience.html