The prime minister selects a large team of ministers to run each of the government departments, such as the Ministry of Defence or the Home Office.
The head of each department is granted membership of the cabinet and is usually described as a secretary of state.
Each secretary of state has a team of junior ministers, known as ministers of state, parliamentary under secretaries of state and parliamentary secretaries.
Each member of the ministerial team is given responsibility for making decisions on a particular area of policy. Ministers take decisions after receiving advice from their civil servants.
Ministers must also keep Parliament informed about the work of their department, appearing regularly in the House to answer oral questions, as well as replying to the many hundreds of written questions that they are asked by MPs or peers.
Most senior ministers are MPs, although some are peers.
The first ministers to be officially termed as such were David Lloyd George, who became Minister of Munitions, and Lord Lansdowne, who became Minister without Portfolio, in 1915.