The components you consistantly find in PBL lessons:
1. Student-centric: Encourges critical thinking, creativity, and real-world experience.
2. Teacher as facilitator: The lesson may start with teacher giving basic instructions and laying ground rules but, as the lesson evolves, students are "released" to pursue the goals as they see fit.
3. Realistic, real-world goals: What the students are expected to learn is attainable given the tools and inforation available, and is relevant to student's experiences.
4. Measurable results: Is there a way to measure what the students are learning, both in formative and summative ways?
5. Variable and wide audience: Can student work be assessed by more than just the teacher? Can it be assessed by topical professionals?