It is the alternative to the Cabinet.
It is made up of shadow ministers. These are the politicians who would take up roles like roles like Chancellor of the Exchequer, Home Secretary and Foreign Secretary if their party was to win a general election and get into Government.
The Shadow Cabinet is part of the Opposition and is led by the Leader of the Opposition. This is the leader of the political party with the second highest number of MPs in the House of Commons. The current Leader of the Opposition is Ed Miliband, who is the leader of the Labour Party.