What is the difference between Catholic and Christian?

All Catholics are Christian, but not all Christians are Catholic. There are many different forms of Christianity, and they vary considerably, but the primary differences are the result of various Christian sects originating in a rejection of Catholicism. 

It is important to understand that for hundreds of years, there was only one form of Christianity, which was Catholicism. Until the Reformation, which was the breaking away from the Catholic Church, to be a Christian was to be Catholic. The Church was a well-organized and well-established religious and political entity.  All power flowed from the Pope and his minions. 

In the sixteenth century came what is now known as the Reformation.  From this time, all those who were Christian and not Catholic began to be known as Protestants, the name rooted in the idea of protest against the Catholic Church.  The Reformation included different sects of Christians, notable Martin Luther, who was protesting the greed of the Church, Henry the Eighth, who wished a divorce the Church would not grant him, and John Calvin, who had his own theological disagreements with Catholicism. Today, these forms of Protestantism are, respectively, Lutheran, Anglican (Episcopalian in the United States,) and the Reformed Church. 

The most substantive difference for all of these and the Catholic Church is a rejection of the authority of the Pope. Other than this difference, there are theological and ritual differences, but each form of Protestantism has its own theology and ritual, some closer to the Catholic Church than others.  As a general rule, those closer to the Catholic Church are spoken of as "high church," and those further away from the Catholic Church are spoken of as "low church."