How the leaders are chosen and how much power the people holds are the factors that shape the government of a nation.
The United States is a democracy, in which the people choose their leaders. But when the Founding Fathers wrote the U.S. Constitution, they specified different ways for choosing the leaders of the three branches of the federal government.
The people directly elect the members of one branch of government. But voters indirectly choose the leaders of the second branch of government. Members of the third branch are not elected at all, but are appointed. Here's a diagram showing how most important US leaders are chosen.
In a dictatorship type of government, one person holds all power (dictator). Usually this person is backed by the military.
Here's a rundown of the various forms of government featured on LiveScience.com.