The following information is from the Research for Better Schools website
http://www.rbs.org/lesson_study/
Many professional organizations promote teacher collaboration, but lesson study goes beyond collaboration to co-planning and observing actual lessons with a focus on student thinking. In the lesson study model of ongoing professional learning, teachers learn together, with administrators and specialists, and even with other schools. Participants plan, observe, and refine "research lessons" designed to make real their long-term goals for student learning and development.
A key, concrete component of lesson study is the observing and teaching of lessons, which then are improved collaboratively. The real "lesson" of lesson study is not product, but process. It compels teachers to examine their own practice in depth in the context of student learning, connects them with their students and their professional community, and inspires them to improve continually. This model of ongoing, teacher-led professional development has been applied most successfully and widely in Japan. It has recently been adapted and initiated by teachers at many sites across the U.S.