Same as the two posters above. Our department head "serves at the pleasure of the Dean," and is NOT a peer-among-equals that a department chair has been in other institutions where I have taught.
This has been my experience as well. Department Heads are delegated authority by the Dean, and serve as the dean's representative to the department, whereas Department Chairs are selected by the department, and represent the department to the dean.
The OP should pay close attention to this. In those examples where an institution has both, appointment of a head - especially an external head - is often either a sign either of a dysfunctional department that needs fixing from above, or of a despotic Dean that is using the appointment as a weapon against a department trying to assert its shared governance rights. - DvF
At my graduate institution, my department had a Head (and had for many years). The more (allegedly) prestigious departments had heads. These were, as mentioned above, appointed by the administration, who then felt obliged to support their boy. Thus, if the Head said, well, we have to do things this way, the admin couldn't argue otherwise without looking foolish (or fearing that they might, which is almost as bad).
The downside is that the admin therefore went out of their way to find Heads who would not tell them things they didn't want to hear. The department finally decided that they feared loss of prestige less than they wanted a chair who would tell the administration what they thought the administration needed to be told. Apparently, with the loss of the Head (and creation of the position of Chair) the administration also somewhat cut the budget and obliged the department to make other changes.
This is my understanding of what happened when I was a grad student, which may be (indeed, almost certainly is) blurred by time and by the reluctance of the tenured faculty to talk about such things in front of the grad students. However, in many respects they did treat us all as "junior colleagues," so perhaps the latter did not play such a great role.