How is the Church of England different from the Episcopalian Church in the United States?
The English monarch is the governor or secular head of the Church of England, which is the established national church of the United Kingdom. Thus clergy in the church of England, in their ordination vows, must swear loyalty to the English monarch. For a United States citizen to have done so after the American Revolution would have been treasonous. Thus the churches in the United States which had, before the Revolution, been part of the Church of England had to separate themselves from the English Establishment and become "free churches", i.e. ones independent of any national government. Once the Episcopal Church had been formed, it lacked bishops, who are necessary to ordain priests and other bishops. Seabury, the first bishop of the Episcopal Church could not have been ordained in England without committing treason, and therefore in 1783 traveled to Scotland, which had a free Episcopal Church (in communion with the Church of England but independent of it), to be consecrated. The Apostolical Succession of all bishops in the Episcopal Church of the USA thus descends directly from the Scottish Church, and the version of the Book of Common Prayer used in the US also is based on the Scottish Prayer Book.