Good teachers have always used projects as a supplement to their regular course of instruction. Any teacher who has taken a group on a field trip, had students enter projects in a science fair, had a class garden, collected and measured the Ph of various water sources, or any one of a thousand activities that involve students in studying and interacting with the real world around them, has conducted a project-based learning activity. Students make decisions, and they have a framework in which to do so. There's a problem without a predetermined solution.
Students design the process for reaching a solution.
Students are responsible for accessing and managing the information they gather. Evaluation takes place continuously. Students regularly reflect on what they're doing. The classroom has an atmosphere that tolerates error and change. Choose what they will work on.