For a description of their job functions go to your Scout Shop and ask for a 
Commissioner Job Description Card BSA No 4265A.  A summary of the BSA 
organizational structure is at:
http://usscouts.org/aboutbsa/bsaorg.html

Steve Henning posted the following good summary:

There are many District positions that
the rest of us can serve in to help these people Deliver The Promise.
These positions include:

Commissioners (Unit Commissioners, Assistant District Commissioners,
District Commissioners) are responsible to provide service to units.  This
service includes rechartering, charter partner relations, problem solving
for units, insuring that Units meet national standards, and providing
information about training, camping  and activities.  All Commissioners
are selected by the District Commissioner, recommended by the Scout
Executive, and approved by the Council Executive Board.

Members of District Committees are responsible to start new units; to
assist with recruiting youth and adult members; raising funds for
recruiting, training and camping facilities; providing recruiting,
training and camping opportunities; providing opportunities for unit fund
raising; and arranging activities.  All members of the District Committees
are elected by the Charter Representatives and Members At Large, and serve
at the pleasure of the District Chairman who is also elected by the
Charter Representatives and Members At Large, and approved by the Council
Executive Board.

The council executive board which is responsible for controlling the
expenditure of council funds and making all policies in the council.  The
executive board is elected by the Charter Representatives and Members At
Large.

The professional Scouters are in place to assist in recruiting, training
and motivating Scouters for all of the other positions.  A professional
Scouter who has done his job properly has nothing to do, since the
volunteer Scouters he recruited, trained and motivated are responsible to
do everything else.  Realistically the professional Scouter spends much
time making sure that all of the various jobs the volunteers are doing are
completed successfully.

--
Cheers, Steve Henning in Reading, PA   USA