A Department is an organizational unit with more than one teacher. The lead teacher, responsible for the work of the department, is the Department Director.
A Unit is a series of Sunday School lessons on a related topic, typically over the course of a month. Each Sunday the class Session contributes toward the unit goal. Unit teaching allows covering more of a topic than a single session can provide; each session typically builds on the content of the previous sessions.
In the department setting, some teaching is best done in Large Group with all the children together (Bible story, reporting on work done, etc..). Some activities are best done in a Small Group setting, with the children divided into several groups with a teacher working with each group for Bible learning activities, Bible research, or other activities which involve the children in discovering Bible truth for themselves.