Separation of powers is the doctrine that each branch of government is separate and has unique powers that the other branches of government may not interfere with. The concept was first formulated by Charles de Secondat, Baron de Montesquieu and is influential in the structure and interpretation of the United States Constitution and the constitutition of state governments of the United States. In the United States the separation of powers is between the Legislative branch the Executive branch and the Judicial branch of government.
For example, the U.S. Congress as the Legislative branch has the sole power to declare war under the Constitution. However, the President of the United States is the Commander-in-Chief of the armed forces of the United States. Therein lies a tension -- Congress may declare war but may not give orders or dictate; the President may give orders or dictate strategy, but may not declare war. Should either branch overstep its limits -- for example, should the President unilaterally invade a foreign state without the authorization of Congress -- the other branch may take steps to correct it. In the above example, Congress might vote to withhold funds from the President or the Supreme Court of the United States may declare the President's acts unconstitutional.
In some instances, legislation might be overturned upon judicial review as violating the separation of powers doctrine. For example, Congress might delegate the authority to make rules to an agency in the Executive branch. If it delegates too much authority and thus attempting to move some of the legislative authority to the Executive branch the legislation might be overturned as unconstitutional as it violates the separation of powers doctrine.
The concept of separation of powers is rejected to some degree by many nations, especially those with parliamentary systems in which the executive is really a subset of the members of the legislature. In the United Kingdom the prevailing governmental doctrine is parliamentary supremacy though under British constitutional law it is limited by various legal instruments and constitutional conventions. As well most democratic nations, including those based upon parliamentary democracy have a concept of judicial independence and this gives such democratic nations a system of checks and balances that is similar to the separation of powers. Constitutional documents such as the English Bill of Rights or the Canadian Charter of Rights and Freedoms also provide limitations on parliamentary supremacy and arrive at the same result as the U.S. concept of separation of powers.
This system was first adapted, in the United States, inorder to prevent what had happened within the Unitary, Monarchy of England. The Monarch could, if he so desired, create laws which only suit his interests, without the consent of the people, or their representatives. Also having both legislative and executive powers, the monarch may choose to create laws, yet not execute them on certain people. For this to happen under separation of powers, the parliament or legislative branch would have to concur indiscriminately with the executive branch.