While I agree with the first answer that power in an autocracy can be seized, that is not the only way that a ruler can get power. There are other ways, especially later on in the life of the autocracy.
The most common (historical) way for an autocrat to get power was to inherit it in some way. Many monarchies have essentially been autocracies. But in these governmental systems, each autocrat did not have to seize power. Someone in the past had seized power and then that power had been handed down through the generations. You can see this in the modern world in, for example, North Korea (not a monarchy, but power has been handed down from father to son to this point).
Power in an autocracy is seized, while power in a democracy comes from an election or from the people. Both are forms of government, often with one ruler. However, the power of an autocrat is unlimited. An example of an autocracy is Nazi Germany. Power in a democracy derives from the governed, so it is limited. The ruler has only that power which the people are willing to give up. An example of a democracy is a town meeting. The United States is a republic. While the United States has many of the elements of a democracy, because we elect representatives who then exercise the right to vote on behalf of the people, it is called a republic, or representative democracy.