The concom runs the convention. The board runs the company: files taxes, allocates profits (if any). (Or "surplus income," I suppose, if it's officially a nonprofit org.)

The concom tends to be comprised of con-experienced fen. Boards tend to be comprised of more-or-less retired fen. (Boards comprised of non-fen tend to kill conventions; conventions are not known for their profit-making abilities.)

The exact relationship between them is different for each con.