There are three primary sources of power for government. These are the consent of the governed, wealth, and force or the threat of force. Sometimes in a given country, it seems as though more than one of these forces is in play.
When the governed consent, we have a democracy. The source of government's power is the people. If they choose to not re-elect a government or to recall a government, the power is no more, and provision is made for an orderly transition of power to a new government.
An oligarchy is a small group of people or families who control the government. The people have had no say in their power. They generally use repressive means to maintain their power, for example, with legislation or by taking over major industries in a country, allowing them to further consolidate their power. Some would point to Russia as an oligarchy, while still others think that the United States is on its way to becoming one.
When power is gained through the threat of violence or through actual violence, we have a dictatorship. Again, the governed are not the source of power. Sometimes the military is engaged against the people, with subsequent threats of further violence. Intimidation results in fear, and the people obey. Assad in Syria is one example, a leader who turns the military loose on those who do not agree with him. Russia, too, has features of dictatorship, with dissidents imprisoned or even murdered for their opposition.
It is difficult to say in some cases that a government is a pure example of just one basis of power. If we look at many countries in the world today, their governments appear to have at least two bases of power.