The Prime Minister is responsible for the overall organisation of the executive and the allocation of functions between ministers in charge of ministerial departments. In general, the ministerial head of a department - usually a secretary of state - has responsibility for a set of policy issues and associated legislation, and these responsibilities are delivered through their department and its delivery partners.
In addition to ministerial departments there are non-ministerial departments and executive agencies. Agencies carry out some of the executive functions of government and are part of a government department, but have their own management and budget.
The responsibilities of ministers and the structure of government are kept under review to ensure they remain fit for purpose. The Prime Minister may wish to reorganise the roles of Cabinet ministers and the Government to reflect changing priorities and challenges. This could involve a combination of reallocating responsibilities between ministers, creating a new department or renaming departments. This is referred to as a machinery of government change.