Original: The Reformed churches are a group of Christian Protestant denominations formally characterized by a similar Calvinist system of doctrine, historically related to the churches that first arose especially in the Swiss Reformation led by Huldrych Zwingli and soon afterward appeared in nations throughout Western and Central Europe.
Simplified: The Reformed churches are a group of Christian Protestant denominations that are historically related by a doctrine that is identical or similar to Calvinism. It developed in the Swiss Reformation led by Huldrych Zwingli and John Calvin, but soon afterwards appeared in nations throughout Western Europe.