PBL is the ongoing act of learning about different subjects simultaneously. This is achieved by guiding students to identify, through research, a real-world problem (local to global) developing its solution using evidence to support the claim, and presenting the solution through a multimedia approach based in a set of 21st-century tools.
Kids show what they learn as they journey through the unit, interact with its lessons, collaborate with each other, and assess themselves and each other. They don't just take a test or produce a product at the end to show their learning.
For more info, you can read this in-depth article through Edutopia.org
