The Royal Arch is the continuation of Craft Freemasonry. Its members, called Companions, meet in Chapters under a Grand Chapter. Chapters are ruled over by three Principals, who rule conjointly, and the Grand Chapter is ruled over by three Grand Principals, with a Pro First Grand Principal when the First Grand Principal is a Royal Prince.
Chapters at home are grouped as a Metropolitan area or Provinces (based on the old Counties) and Chapters overseas are grouped in Districts. Metropolitan, Provincial and District Grand Chapters are ruled over by a Grand Superintendent who is appointed by the First Grand Principal as his personal representative for the particular area.
In England the Royal Arch has four ceremonies: the exaltation ceremony to bring in new members and an installation ceremony for each of the three Principals. The exaltation ceremony is in two parts: a rather dramatic presentation of the principles of the Order followed by three Lectures in which the history, symbolism and principles of the Royal Arch are further explained. Like Craft Freemasonry, the Royal Arch is open to men of all faiths.
The allegory of the exaltation ceremony is based on the Old Testament telling of the return to Jerusalem from the Babylonish captivity to rebuild the city and temple. In clearing the ground of the original temple for the foundations of the second temple, the candidate makes a number of discoveries which emphasise the centrality of God to man's life and existence and, without transgressing the bounds of religion, lead the candidate to a consideration of the nature of God and his personal relationship with Him, whatever his religion might be.
In England, the Royal Arch is considered to be the completion of "pure ancient Masonry". In the Craft the candidate is presented with a series of eminently practical principles and tenets which if he practises them he may hope to live a life pleasing to his God, however he worships him, and of service to his fellow man. But man is not simply a practical being, he has an essential spiritual aspect to his nature. That spiritual aspect is introduced in the Third Degree, in which the candidate is led to a contemplation of man's inevitable destiny, and becomes the central message of the Royal Arch. In that sense, "pure ancient Masonry" can be seen as a journey of self - knowledge and discovery with the Royal Arch completing the practical lessons of the Craft by a contemplation of man's spiritual nature, not replacing but reinforcing and supporting what he has learned from his religion.
From the Grand Chapter web site