A teacher diary is a diary where a teacher records what happens in their classes and their thoughts about it. Teacher diaries are used as development tools.
After a class that went badly, the teacher makes notes in her diary about what happened, what she thought the causes were, ideas about how to change them and a short action plan.
In the classroom
Teacher diaries are a good way to start a course of development; a natural second step would be peer observation, i.e. getting another teacher to analyse the class in the same way.