What is federal government?
In federal states, the federal government is the government at the level of the nation-state. The structure of central governments varies from institution to institution. By delegating powers from the central government to governments at a sub national level, an autonomous regions are created by many countries. Based on a broad definition of a basic political system, there are two or more levels of government that exist within an established territory and govern through common institutions with overlapping or shared powers as prescribed by a constitution or other law.
As a basics, the central government has the power to make and enforece laws for the whole country which are in contrast with local government. The usual responsibilities of the central government which are not granted to lower levels of government are maintaining national security and exercising international diplomacy, including the right to sign binding treaties.
In general, the difference between a central government and a federal central government is that the autonomous statuses of self-governing regions exists by the sufferance of the central government and are often created through a process of devolution. It is common for a federal central government to be brought into being by agreement between a number of formally independent states and therefore its powers to affect the status of the balance of powers is significantly smaller. For example are the United States of America.

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What is unitary government?
When the central government possesses much authority and decision-making power, it is called the unitary government. The local governing bodies simply serve as administrative arms of the central government. For an example of a unitary government are the Great Britain which is a familiar example of a unitary government; individual British counties have little of the power commonly exercised by American states. France also has a unitary form of government where by it has 90 departments grouped into 36 provinces. It is important, however, to note that unitary governments are not inherently less democratic than other forms.
Power is distributed completely opposite of a unitary government in a confederate government. Local governments protect and preserve their own authority by forming a weak central government.
In a federal government, power is split between a central government authority and its constituent states. Usually, a constitution allocates duties, rights, and privileges to each level of government. The constitution usually defines how power is shared between national, state, and local governments; the power to amend this constitution is usually granted to the citizens or their governmental representatives.
There are two basic categories in which governments are classified according to a decision making. In a totalitarian government, the power of rulers is not limited by outside forces, such as elections or public opinion. Totalitarian systems also restrict personal freedom in most cases.
How does unitary government differ from a federal government?
In a unitary government, the power is held by one central authority but in a federal government, the power is divided between national government or federal government and local governments or states government.
Federal government

has multiple hierarchy levels, with both the central authority and the states (or provinces) both being sovereign. 
the central or national rules override the state rules
has a balance between them. For example are the U.S

Shared between national and local levels. In a federal form of government, the term “federal” is also used to refer to the national level of government.
Unitary government

there is no hierarchy of sovereign powers.
states have no authority to pass their own laws, and the central or national govt can order the states to do anything.
the federal government has a huge percentage of the power.

What are the Advantages and Disadvantages to a Unitary System of government?
A unitary government is a government where the vast majority of authority is held by the federal government.
Advantages of Unitary System
The advantages of unitary government are it is single and decisive legislative. Usually it is more efficient in the used of tax dollars but fewer people trying to get in on the money. It is also has a simple management of an economy and the government are smaller. 
Disadvantages of Unitary System
The disadvantages of using this type of system are it has slow government response. For example, there are no state National Guard that could be dispatched in emergency, troops would have to be mobilized from national authority. It is also easily looses track of local issues. Other than that, it is incredibly divisive form of government where everyone is forced to compete with everyone else for priority. Since it is trying to take the place of federal and state governments, the unitary governments typical get bloated and bogged down. Finally, it has huge bureaucracy that is even larger than what this country has.
What are the Advantages and the disadvantages of Federalism?
Advantages of Federalism
Every province has political, social and economic problems peculiar to the region itself. Provincial government representatives live in close proximity to the people and are most of the times from the same community, so that they are in a better position to understand these problems and offer unique solutions for them. For example, traffic congestion in Oahu, Hawaii is a problem that can be best solved by the local government, keeping local factors in mind, rather than by somebody living in New York.
Federalism offers representation to different populations. Citizens of various provinces may have different aspirations, ethnicity and follow different cultures. The central government can sometimes overlook these differences and adopt policies which cater to the majority. This is where the regional government steps in. While formulating policies, local needs, tastes and opinions are given due consideration by the state governments. Rights of the minorities are protected too. For example, in states like Arizona where there is a large Hispanic population and therefore, a large number of schools provide bilingual education.
State governments have the freedom to adopt policies which may not be followed nationally or by any other state. For example, same sex marriages are not recognized by the federal government of USA but they are given legal status within the states of Connecticut, Iowa, Vermont and Massachusetts.
Division of work between the central and the regional governments leads to optimum utilization of resources. The central government can concentrate more on international affairs and defense of the country while the provincial government can cater to the local needs. 
Federalism has room for innovation and experimentation. Two local governments can have two different approaches to bring reforms in any area of public domain, be it taxation or education. The comparison of the results of these policies can give a clear idea of which policy is better and thus, can be adopted in the future.
Disadvantages of Federalism
Sharing of power between the Center and the states includes both advantages and disadvantages of federation. Sometimes there can be overlapping of work and subsequent confusion regarding who is responsible for what. For example, when typhoon Katrina hit Greater New Orleans, USA, in 2005, there was delay in the rescue work as there was confusion between the state governments and the federal government on who is responsible for which disaster management work. This resulted in the loss of many lives.
The federal system of government is very expensive as more people are elected to office, both at the state and the center, than necessary. Thus, it is often said that only rich countries can afford it. Too many elected representatives with overlapping roles may also lead to corruption.
Other than that, it leads to unnecessary competition between different regions. There can be a rebellion by a regional government against the national government too. Both scenarios pose a threat to the countries’ integrity. 
It is also promotes regional inequalities. Natural resources, industries, employment opportunities differ from region to region. Hence earnings and wealth are unevenly distributed. Rich states offer more opportunities and benefits to its citizens than poor states can. Thus, the gap between rich and poor states widens. 
It also can make the state governments selfish and concerned only about their own region’s progress. They can formulate policies which might be detrimental to other regions. For example, pollution from a province which is promoting industrialization in a big way can affect another region which depends solely on agriculture and cause crop damage.
Finally, it does not eliminate poverty. Even in New York, there are poor neighborhoods like Harlem with a majority of black population. The reason for this may be that during policy framing, it is the intellectuals and not the masses who are invited by the local government.