The Themes and Units category will encompass planned themes for a period of time and units of study in a particular subject.
Themes are more general. For example we might choose bears as a theme and will have books about bears, counting bears for math work, art projects with bears, etc.
A unit would be more academic in nature such as a unit of study on clouds. We would research, read books about clouds, use art to explore clouds, do experiments, etc. It is a bit more focused.