I think the first thing you should do is visit
mosforge.net and then
mamboportal.com to look over the lists of available modules/components and what they are for. They are certainly too numerous to mention here. The "must have" modules for me are the flash clock/calendar, events calendar and last five SB posts...plus so many more. It really depends on what you want to offer.
As far as DocMan is concerned...it is a component for listing available documents or files by category. It allows for a description of the file and basically serves as a database for the user to browse through to find files/documents. Remository is similar, but provides more file details and is more suited to file management where DocMan is more suited to document management.