Project-based learning focuses on how to answer a question that children have about the world. It requires them to work in groups to decide on how they want to answer the question and what resources they will use. By using project-based learning, teachers and homeschooling parents of multi-age children are letting children taking the initiative in their learning.
This is an infographic that I've written over at Muslimah Bloggers. Head on over to read about project-based learning in more details.