The Spacing Guild is one of the three Great Schools of the Dune universe, written by Frank Herbert. At the time of the novels, the Spacing Guild is one of the three forces controlling the government of the Imperium. It does this via controlling all travel between worlds as well as all banking.
The Spacing Guild's leaders and adepts are the Navigators, who possess the talent to safely guide a Heighliner through foldspace. The Guild is always searching for new additions to the ranks of the Navigators, those who can be one with the universe. There are also a number of "normal" employees of the Guild, who act as bankers and low-level ambassadors to the Imperium.
The Guild also often acts as an enforcer of peace in the Imperium. They charge outrageous fees for the transport of military troops and may in some cases refuse such a transport when they feel the need. Also, the Guild's primary threat of an embargo upon a planet serves to curb great atrocities, for without the Guild's transport, a planet is completely isolated from the Imperium.
Unfortunately, the Spacing Guild has one tragic weakness; it is wholly dependent on the spice melange. The Guild Navigators all exist within tanks filled with melange and are completely addicted to it. Additionally, their talents of prescience are bound to the spice. Since melange is only available on the desert planet of Arrakis, whoever controls Arrakis controls the spice, and thus the Guild...something which they learned the hard way when Paul Atreides seized control of Arrakis.