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.