Some of the posts in this thread seem to be about the Head Server, while others are about the Dining Manager. These are two different positions.
Also, I've seen many cases on this board in which someone refers to their Server as their Head Server (in contrast to their Assistant Server).
Here's what the various people do:
The Assistant Server takes and delivers drink orders, assists the Server, and often takes away plates between courses.
The Server takes and delivers your food orders, and works closely with your Assistant Server as the leader of the 2-person team.
The Head Server is responsible for a section of the restaurant and for the Servers and Assistant Servers in that section. The Head Server should make sure that everything is going properly. Ideally, you won't have problems that require the Head Server beacuse the Head Server has dealt with everything before you even become aware of it. Also the Head Server is likely to greet you most evenings and make sure you're happy. Just like other servers, the Head Server works 7 days a week, usually from early morning until late at night (with some time off during non-meal periods during the day). The Head Server essentially earns his or her income from tips.
The Dining Manager is a person with whom most guests never have direct interaction. See the explanation from frakers in this thread. I have no idea who the Dining Manager was on our
DCL cruises, and the Dining Manager did not come around for a tip on the final night. (I imagine Chattyaholic was really writing about her Head Server, not the Dining Manager.)