Usually, there are priests, then there are bishops. However, some Protestant churches have no bishops or archbishops. The Presbyterian churches are examples. The Church of Scotland is headed by a Moderator, who is elected each year by the General Assembly each year. Other Christian movements have neither bishops nor priests: Quakers are a good example.
In the Catholic church, the Pope is chosen by all the cardinals from amongst their number. According to church law, this does not have to be the case: any male, unmarried, baptized Christian who is judged fit for the office can become pope. However, the last pope who was not a bishop was Urban VI (elected in 1378).
The pope is also the called 'the Bishop of Rome'. In fact he rules an independent state within Rome, called the Vatican. All Roman Catholic bishops answer to the pope (or to patriarchs in some orthodox churches). In the Anglican church, bishops are governed by Archbishops.