A monarchy is a form of government that has a single person known as a monarch at its head. Monarchs use such titles as king, queen, emperor, or empress. Also, most monarchies are hereditary. This means that when the monarch dies, a son, daughter, or other relative becomes the next monarch.
A dictatorship is a form of government in which a person or a small group rules with almost unlimited power. The ruler of a dictatorship is called a dictator. Dictators take power by force or by misleading the people.
Oligarchy is a form of government where most political power effectively rests with a small segment of society (typically the wealthiest or most ruthless elite). The Greek etymological origins of the word "oligarchy" express the concepts of "few" and "rule".
The word democracy literally means “rule by the people.” It is derived from a Greek word coined from the words demos (“people”) and kratos (“rule”) in the middle of the 5th century bc as a name for the political system that existed at the time in some of the cities of Greece, notably Athens.
Theocracy is a form of government in which the governmental rulers are identical with the leaders of the dominant religion, and governmental policies are either identical with or strongly influenced by the principals of the majority religion. Typically the government claims to rule on behalf of God or a higher power, as specified by the religion in question.