The Combine Honnete Ober Advancer Mercantiles or CHOAM, essentially controls all economic affairs across the cosmos, although it relies upon the Spacing Guild for transport across space due to the Guild's monopoly on faster-than-light travel.
Many Houses depend on CHOAM profits, and an enormous proportion of those depend on melange.
CHOAM directorships were the real evidence of political power in the Imperium, passing with the shifts of voting strength within the Landsraad as it balanced itself against the Emperor and his supporters. Directorship in the CHOAM was the key to wealth, each noble House dipping from the company's coffers whatever it could under this power. The flag of CHOAM is yellow, blazoned with a circle, half-red and half-black.
The corporation's management and board of directors are controlled by the Padishah Emperor and the Landsraad (with the Spacing Guild and the Bene Gesserit as silent partners). Because of its control of inter-planetary commerce, CHOAM is the largest single source of wealth in the Old Empire; as such, influence in CHOAM (through partisans within it and control of directorships) is the central goal of political maneuvering, both to receive dividends and also (it is implied) to skim off profits.
The structure of CHOAM is similar to that of a publicly-held corporation. It consists of shareholders, and, as in a public company, major shareholders are given directorships to lead in the board of directors. In this case, however, all shareholders are nobles from the Landsraad, which consists of the Minor Houses, the Great Houses and the Imperial House. The Great Houses predictably hold the directorships of the company, but the Emperor is able to give out directorships and revoke directorships at whim.
God Emperor and Afterwards
Under the God Emperor Leto Atreides II, CHOAM experienced a reduction of influence and power, just as every other organization did under his reign. CHOAM survived in the regions of the Old Imperium during the Scattering and during the Return of the Honored Matres.
Behind the Scenes
As stated by Frank Herbert, the CHOAM company works in a similar way to OPEC - that is, controlling, exploring vital products to enrich Great Houses.
CHOAM was ranked by Forbes magazine as the largest fictional corporation.