Sovereignty
Sovereignty is a state's supreme authority to govern itself, free from external control -- both internally (having the final say over its own laws and affairs) and externally (recognized as independent by other states in the international system).
Power vs authority
Power is simply the ability to make others act a certain way, whether or not they accept it as right (e.g. a dictator ruling through fear still holds power). Authority is power that is seen as legitimate and rightful by those subject to it -- people obey not just out of fear, but because they accept the ruler's right to command.
Example 1: A newly installed military ruler forces compliance through the threat of the army, but citizens widely see the takeover as illegal and resent the regime. Does this ruler have power, authority, or both?
This ruler has power (the ability to enforce compliance through force) but lacks authority, since citizens do not recognize the takeover as legitimate or rightful -- illustrating that power and authority, while often found together, are not the same thing.
Forms of government
A democracy places power in the hands of the people, either directly or through elected representatives, typically with regular free elections, protected rights, and government accountability. An autocracy concentrates power in a single ruler or small group with little accountability to the wider population. A totalitarian government goes further, seeking to control nearly all aspects of public and private life, often suppressing opposition and dissent entirely.