A city charter is a set of laws that forms the foundational governance structure for a city. City charters stand in the same relationship to a city that a state constitution does to a state, or a federal constitution does to a nation.
Not all states allow the formation of cities with charters. In those states that do, some cities under some conditions are allowed to become so-called "charter cities", while the non-chartered cities are referred to as "general law cities." General law cities are governed under whatever state constitutional provisions in a particular state lay out fundamental governance doctrine for cities, while charter cities are governed under their own charter.
As of Aug 2010, 119 of California's 478 incorporated cities had become charter cities.