Separation of Powers means that the three branches of government are separated.
The three branches are as follows:
- the Legislative - the part that makes laws
- the Executive - the part that carries out (executes) the laws
- the Judicial Branch - the courts that decide if the law has been broken
Separation of Powers helps to protect freedom. The executive branch carries out the laws but cannot make laws to make themselves powerful. Also the judiciary is responsible for making sure that criminals are punished, so that members of the government or legislature cannot ignore the law as the judiciary can check on them.
Separation of powers is also called a system of checks and balances because the branches can check up on each other and if any of the branches get too strong, that branch will be balanced by the others.
In the United States the three branches of government are completely separate except for the Vice President who is President of the Senate. In the United Kingdom the three branches of Government are mixed but the checks and balances are provided by history and custom (the rule that says something should happen because that is how it has been done for a long time). The Queen is Head of State (the executive), but is also part of Parliament (the legislative branch) and is the Fountain of Justice (the head of the judicial branch). But by convention she does not do anything without the advice of Ministers and never refuses to pass an Act of Parliament. The Queen has a lot of power but the power is controlled and balanced by the need to act in certain ways or only use the power at certain times.
In some countries the leaders of the executive branch are members of the legislature. This system is called responsible government. The first to talk about separation of powers in the modern age was Charles-Louis Montesquieu. Montesquieu published his book De l'esprit des lois (The Spirit of Laws) in 1748.