Ministers make up the Government and are almost always Members of the House of Commons or the House of Lords. There are three main types of minister: Departmental Ministers, Ministers of State and Junior Ministers. Departmental Ministers are in charge of government departments. The Government is divided into different departments which have responsibilities for different areas. For example the Treasury is in charge of government spending.
Departmental Ministers in the Cabinet are generally called 'Secretary of State' but some have special titles such as Chancellor of the Exchequer. Ministers of State and Junior Ministers assist the ministers in charge of the department. They normally have responsibility for a particular area within the department and are sometimes given a title that reflects this.
Further information on this subject can be found from the following links.
Her Majesty's Government
About Parliament: Parliament and government
Parliamentary Information List: Ministerial departures since 1997 ( PDF 85 KB)