Separation of powers is a doctrine of constitutional law under which the three branches of government (executive, legislative, and judicial) are kept separate. This is also known as the system of checks and balances, because each branch is given certain powers so as to check and balance the other branches.
Each branch has separate powers, and generally each branch is not allowed to exercise the powers of the other branches.
The Legislative Branch exercises congressional power, the Executive Branch exercises executive power, and the Judicial Branch exercises judicial review.
The premise behind the Separation of Power is that when a single person or group has a large amount of power, they can become dangerous to citizens. The separation of Power is a method of removing the amount of power in any groups hands, making it more difficult to abuse. It is generally accepted that there are three main categories of governmental functions: (i) the legislative, (ii) the Executive, and (iii) the Judicial. At the same time, there are three main organs of the Government in state i.e. legislature, executive and judiciary. According to the theory of separation of powers, these three powers and functions of the Government must, in a free democracy, always be kept separate and exercised by separate organs of the Government. Thus, the legislature cannot exercise executive or judicial power; the executive exercise legislative or judicial power of the Government.