Each member of the Board of Supervisors is elected from a Supervisorial District.
Districts
The districts are determined in part based on Census data. Every ten years (after a Census), the city must determine if its districts "meet the legal requirements established by federal, state and local law." The City Charter also plays a part in redistricting, as it states that population variations between districts should be no more than one percent of the statistical mean “unless additional variations, limited to five percent of the statistical mean, are necessary to prevent dividing or diluting the voting power of minorities and/or to keep recognized neighborhoods intact; provided, however, that the redistricting provided for herein shall conform to the rule of one person, one vote, and shall reflect communities of interest within the City and County.”
The San Francisco Redistricting Task Force is responsible for coming up with a redistricting plan which outlines new district lines every ten years. The Redistricting Task Force presents its report to the Board of Supervisiors.
2012 Redistricting
The Redistricting Task Force working draft for 2012 redistricting, 2012-2-6
Some info here.
