Simply put, a Theocracy is any society in which the The Church
is the government. Often the laws of a theocracy are based off religious law, or claims that God (or Gods) is the supreme ruler of their state. This is especially prevalent in pre-modern settings.
It's common to have an official state religion, but this doesn't necessarily equate to a theocracy or even an especially religious country. For example, in England the head of state (the monarch) is also the head of the Church, bringing an overtly religious aspect into the governmental system, but England and the UK in terms of population are much less religious than nearby, officially secular Ireland and France.
Note that true theocracies, where secular government is virtually non-existent, are fairly rare. Most often the Church will simply have a lot of secular power and sometimes a parallel government: authority over religious/moral laws, it's own bureaucracy, it's own army, etc.
Compare Church Militant
, where the clergy is Badass
, but not necessarily the rulers of a country. A Corrupt Church
is often the head of a Theocracy, but not always.