Hiya,.,...
Well a CSD is basically the overall incharge crew member third in the chain of command (after the two pilots). Its his/her job to delegate all the crew and also be leader in the event of any problems. They are also incharge of all the paperwork that is required in the day to day operation of a flight.
Unfortunately there are a few muppets out there who see the position as a step towards retirement and tend not to do very much other than pretend to look busy and shake hands with the frequent flyers, oh, and lets not forget the all important task of handing out the GPM's to random passengers, its amazing how long that can take with some people. On longhaul aircraft like the 777 and 747 they even have there own little office and its not unusual to see them sitting there for the entire flight!!! Fortunately these muppets are few and far between and the majority of CSD's are true leaders with excellent interpersonal skills and experience and are a pleasure to work with.