I'm going to assume in the US because that's where I live.
The US is a group of 50 governments that came together and also formed another, larger government for the whole country. These 50 smaller governments are "States" and the one bigger government is the "federal" government.
Both levels of government have different powers that only they can do. This concept is called federalism.
State governments obviously only make laws that affect their own state. States do most of the governmental things that people come in contact with on a daily basis. They are responsible for most roads, schools, police, etc.
The Federal government does things that state governments cannot do. They make laws that apply across the whole country and control the US military. The federal government is bound by the Constitution.
The Constitution is a document that sets up how the federal government will work. It is an instruction manual and rule book for the federal government. It was made in 1787 and has been changed, or "amended" 27 times since then. The Constitution separates the federal government into three parts, or "branches:" Congress, the Executive, and the Courts.
The word "Congress" comes from the word "congregate" which means to come together and discuss things. This is because Congress is where each of the 50 states sends people to make laws.
Anything done by the federal government is caused by laws*, and all laws are made by Congress.
Congress has two groups in it: the House of Representatives, and the Senate. Both groups need to pass a law before the law can happen. The Senate has two members from every state, and the House has different numbers from every state depending on how many people live in that state.
The word "executive" comes from the word "execute," which means to make something happen. So the Executive branch of government, headed by the President, makes all the laws passed by Congress happen.
The Executive branch is the branch that actually does everything outside of Washington DC. It collects taxes, spends money, orders the military, etc.
The President is also responsible for dealing with other countries.
The courts are also known as the "judicial" branch, which means "to decide." This is because the Courts decide what happens when two people disagree about what a law means. The Courts say who is right and how a law should be applied.
There are 3 different levels of federal courts. If someone does not agree with what the first level says, they can take the case to the next level up. The highest court is called the Supreme Court. There is not a higher authority in the government than the Supreme Court.