What is 'lesson study'?

See this longer Washington Post article for a case study from the USA: Ensuring No Teacher Left Behind Professional Development Model Gets Inside Students' Minds

Third-grade teacher Andy Gomez stood at a whiteboard before 10 of his colleagues on a recent Thursday afternoon at Marie Reed Elementary in Adams Morgan... For the next half-hour, the group discussed -- down to nitty-gritty details about vocabulary to use or avoid -- what the students' fundamental misunderstandings about numbers might be and how to address them...This collaborative examination of the mechanics of teaching is part of the school's embrace of "lesson study," a model of professional development for teachers that was developed in Japan...It is a wholly different approach from the workshop-with-an-outside-expert model that dominates professional development... A 2006 study showed that the increase in test scores over four years at a California school where teachers engaged in lesson study was triple that of other schools in the same district... "Lesson study is a way for teachers to get better. It provides a vehicle to grow,"... Education researchers James Stigler and James Hiebert first popularized lesson study in the United States in 1999, when they published the book "The Teaching Gap," in which they compared education in cultures around the globe. They described a Japanese system in which teachers are constantly examining and tweaking their practice rather than attempting wholesale reform, as has failed so many times in America... "The evidence is pretty good that the only kind of improvements in teaching that are going to be sustainable are going to be small, incremental improvements".