What is the difference between academic and administrative department heads?
An administrative head of department heads up the administration of the department, and will typically have little or no involvement in the direction or the content of the research or teaching.

An academic department head leads the academic direction of the department. This often include somes administrative duties. Well, they nominally lead the academic direction of the department. In theory, they guide the direction of the department's researchers and teaching. In practice, that's like herding cats.