Legislative branch means the branch of government which is concerned with making of laws. The President who is the head of the State and Commander-in-chief of the armed forces is vested with the executive power i.e., implementing the laws. The President can also veto legislation created by Congress and nominate heads of federal agencies. Veto means a Constitutional right to reject a decision or proposal made by a law-making body. In this manner laws that require to be revisited for necessary reasons are sent back to the law-making body by the executive.
Executive branch means the branch of government that oversees the carrying out of laws meaning it sees that the laws are executed well. This branch is concerned with executing the laws because just making laws is not sufficient but the important aspect of law-making is to make it effective and seeing the law being carried out for the common goof of all. The Congress which is the law-making body in United States of America can confirm or reject President’s nominees and can remove the President in exceptional circumstances. Thus, the executive head cannot take advantage of the position or misuse the executive powers.
Judicial branch is concerned with the administration of justice. This is done by interpreting the laws that are passed by the Congress. The Justices of the Supreme Court can overturn unconstitutional laws. The Justices are nominated by the President and are confirmed by the Senate. In this manner all the three branches are independent yet the power and authority is not absolute so that there is no misuse of the powers that are granted to the branches. Each branch keeps a check over the functioning of the other branch despite there being separation of powers among the branches.
In simple words federalism is when two sets of Governments namely Central and regional operate and function simultaneously.
Some of the countries where federal system is seen is in United States of America, Republic of India etc. where the power, authority and responsibility is divided between two sets of Governments namely that of the States and of the Center.
A written Constitution is necessary when there is federal system so that the respective jurisdictions of the Governments are specified in-order to avoid any conflict related to the power, responsibility and authority between the Governments.
Each set of Government requires its own jurisdiction in specific matters of legislative, financial or any administrative subjects so that the States too have their respective opinions, interests and needs represented equally.
In the federal system the fundamental principles laid down in the Constitution cannot be unilaterally changed by any Government.