A Rogue Trader is a combination of freelance explorer, conquistador and merchant. They are Imperial servants, given a ship, a crew, a contingent of Marines or Guardsmen and carte blanche to roam the worlds beyond Imperial control.
Many times Rogue Traders find isolated planets with a Human population which will be incorporated into the Imperium. Other times they find empty or alien-dominated planets ripe for colonization. Rogue Traders are flamboyant individuals; many are highly pious while others are no more than legitimized pirates. The most infamous Rogue Trader was Jan van Yastobaal, who became little more than a desperado plundering whatever world he came upon.
The most valuable possession of a Rogue Trader family is its Charters, or Warrants of Trade; an ancient legal document which describes the accepted limits of its operations. These charters are hereditary thus creating a Rogue Trader dynasty.
Certain powerful lords of the Adeptus Terra offer charters of trade as a bribe to their rivals. In exchange the rival simply walks away into a life of adventure and, hopefully, great wealth. Some Rogue Trader families have powerful allies inside the Adeptus Terra.
Some of the more ancient ones are dated from the very beginnings of the Imperium, and some were even signed by the Emperor himself. Others were signed by Primarchs or other leaders of the Great Crusade. These charters are very valuable and give its owner great leeway. They are tailor-made and unique. They cannot be re-appealed as according to Imperial law and Imperial religious dogma no one is empowered to overrule these persons.
Having said that, one can assume that a renegade Rogue Trader guilty of heresy or treason will be severely prosecuted by the Inquisition. It is very likely that he is considered to have forsaken the Emperor's trust upon his family and that his punishment will be much more severe.
Letters of Marque are similar documents issued more recently. The Letters of Marque are rather restricted in some aspects and controlled more effectively by Imperial authorities. The more recent ones are not hereditary at all, forcing the hopeful heir to return and re-apply for a new charter.
A Charter of Trade or a Letter of Marque may contain peculiar restrictions. This may be of geographical nature (e.g.: restrict the Rogue Trader into a single sector) or something similar.
Some Rogue Traders are relatively poor, possessing a single ancient and dilapidated ship. Others are incredibly wealthy and powerful and have whole fleets and entire private armies at their disposal. Certain favored individuals may even call upon detachments of Space Marines having entered pacts with individual Chapters. Some Rogue Traders operate as mercenaries, renting the service of their ship(s).
Depending upon the families wealth, power, and connections it can be considered an aristocratic family.