Rejoice For You Are True
The Emperor’s Priesthood and the administers of the Imperium. The Lords of Terra are the nominal rulers of the Imperium while the Emperor is unable to make his will known through normal channels.
Nominally, the Adeptus Mechanicus is also part of the Adeputs Terra, although they wield far more power than the other sub-groups.
In some byzantine fashion, the Adeptus Terra shares authority with the Adeptus Ministorum. A normal Imperial citizen, and in fact most movers and shakers, really have no understanding of how power is shared out through the immense bureaucracy.