The City Council is the legislative arm of the City of Seattle, responsible for passing and amending the city’s laws. (The Mayor heads the Executive Branch)
The City Charter establishes the City of Seattle and its general functions (the equivalent to the state or federal Constitution).
The Municipal Code is the body of laws that the City Council are responsible for.
The City Council is also responsible for writing and passing a budget. As with the state and federal government, the head of the executive branch (in this case the Mayor) submits a budget, which is a starting point for the council’s further edits.