The original or official
Traveller universe is centered on a part of the local arm of the Milky Way known as
Charted Space. There are several large and scores of small political entities in Charted Space, centered on the Third Imperium, the largest human-dominated polity.
The Third Imperium controls its eleven thousand systems indirectly; most planets are under local control, paying taxes to the Imperium to maintain security which the individual worlds would be hard-pressed to provide for themselves. In exchange for paying for the Imperial Navy and observing some basic laws, the Imperium cedes a great deal of autonomy to the local governments. The result is that each star system in the Imperium may differ considerably from the others, in terms of government system, legal strictures, and technological development.
A good deal of what the Imperium does is designed to keep trade flowing between worlds, from the Imperium-spanning megacorporations down to planetary businesses and individual 'free' traders.
One of the reasons for limited Imperial control and a high degree of local autonomy is that the speed of communications is limited to the speed of ships moving through space; there is no faster-than-light communication, so a simple question about local events may take weeks months to travel from the capital to the border regions and back again. A local bureaucracy led by an Imperial noble is granted a great deal of leeway to deal with problems in its area of control, under guidelines from the Emperor. This impacts megacorporations the same way; local managers work on general guidance from the board of directors who have little day-to-day or even month-to-month control over operations.
The local Imperial nobles are not above intervening in that local autonomy, however; for example, if a conflict between two systems turns into a hot war, or if some important Imperial interest is threatened by local action, then the Imperium may intervene, covertly (perhaps by hiring a mercenary troop through a front company) or overtly (no one wants to be on the receiving end of battle-dressed Imperial Marines backed by an Imperial Navy dreadnaught squadron).
The Third Imperium is mindful of its internal and external security because it is surrounded by dangerous polities, from the human psionic Zhodani and racist Solomani to the alien piratical Vargr, genocidal K'kree, and subversive Hivers.
This makes the Third Imperium a barely civilized place, where the Emperor's reach is only as long as the arm of his fleet, where megacorporations mount low-intensity (as long as you're not where the shootin's goin' on) wars over control of markets and resources, where Imperial nobles vie for greater power and wealth, and where the resident sophonts are struggling to make a credit in a system rigged against them, and to stay alive while doing it.
This is why the Third Imperium is one of my favorite roleplaying settings ever.
