To understand how a country is run - it's useful to have a clear picture of the main institutions of the State. In most countries, institutions serve the different functions of government. The analysis of the work of government into making law (Legislative Function); carrying out the administration of government - delivering services & providing internal and external security (Executive Function); and judging on the interpretation of law and dealing with legal disputes and judging whether there has been a breach of the criminal law (Judicial Function) - can be traced back to Ancient Greece.
The MindMap below stresses the main features of the UK State Institutions. Remember that many functions have been devolved to institutions serving Wales; Scotland and Northern Ireland.