This term is a little ambiguous. If you weren't looking for the profession, check the bottom of the page for other uses.
In the Warhammer 40,000 universe, Rogue Traders are like the conquistadors and explorers of the Age of Sail -- they go into the unknown to find new worlds for the Imperium, and hopefully get rich doing so. The existence of the traders is a Faustian bargain the Imperium has been making since the days of the Emperor himself. The High Lords of Terra recognize that there are still many parts of the galaxy (and many situations therein) that are beyond their control. By allowing the Rogue Traders to ply their trade with planets and even species outside of the Imperial peace they can maintain some tenuous influence over events there. The traders have almost complete discretion in their dealings, and are allowed to do almost anything so long as it can be seen as advancing Imperial goals. In this sense traders have incredible power second only to that of an Inquisitor.
Of course it's a bold and foolish captain who pushes this freedom too far. HERESY!*BLAM*
The Imperium has no great love for the traders, and those who pursue their own agenda or flaunt their deviancy too openly tend to come to a bad end. That said, the fact that no one save the highest ranks of the Imperial hierarchy exerts any official influence over Traders means that bringing them to heel often requires fairly covert means, up to and including recruiting other Rogue Traders to do the dirty deed.
Essentially smooth criminals in space.
The Warrant Of Trade
This being the grimdark far-future and all, not EVERYBODY can have "rights" or be given "freedoms". As such, the only thing separating a Rogue Trader from a plain ole' rogue is the Warrant Of Trade, a document written by the Emperor himself that gives the holder of the document the very rare liberty of travelling outside of Imperial space, trade with xenos, and even set up their own pocket-empire, as long as they officially remain a part of the Imperium and pay their taxes. Warrants Of Trade are also hereditary, meaning the scion of a Rogue Trader will inherit that power and so forth, creating a Rogue Trader dynasty. The granting of a Warrant usually includes one or two conditions, such as visiting some backwater rock in the vast emptiness of space, or lending aid to one of the countless wars the Imperium got itself caught up in. If a Warrant is granted without any conditions, that means the High Lords Of Terra hope the Rogue Trader will go fuck off and get NOMMED on some isolated shithole of a planet. Conversely, as mentioned above issuing a new Warrant of Trade is a good way for the High Lords to get over bureaucratic inertia. If some planet somewhere really needs conquering ASAP, it's often much easier to make a newly-invested Trader do it than go through the long, slow process of mobilizing the Imperial Guard.
Rogue Traders and Xenos
As the Imperium's primary means of making first contact with unexplored or forgotten parts of the Galaxy, Rogue Traders are often put in the strange position of being Imperial emissaries to species they are, doctrinally speaking, supposed to exterminate. The semi-independence afforded by the Warrant of Trade often means, however, that Traders are much more willing to cooperate or at the very least grudgingly tolerate xenos, whether for the sake of profit or because having a crew of, say, Ork mercenaries at your side at all times makes killing you all the more difficult. This also makes them the center of what is known as the Cold Trade in smuggled xenos artefacts and technology.
While trading in xenotech is, of course, incredibly illegal in the Imperium, the industry persists firstly because xenotechnology tends to be an insanely valuable commodity amongst those with the money to afford it, and second because the Adeptus Mechanicus is always looking for new xenotech and archeotech to study. The Cold Trade thus supplies rich hive spirers with fun alien guns to use on poor people, as well as allowing the Mechanicus to discover new ways to horribly murder dangerous xenos species. Everybody wins!
Expanding Imperial Borders for Fun and Profit
Though the classic image of the Rogue Trader has them out amongst the unknown stars, swashing buckles and buckling swashes, many make their fortune well within the boundaries of Imperial space, looking to exploit resources already settled by Man. This can be as simple as surveying planets for resources, or even creating full-fledged Imperial colonies (for a cut of the profits, of course). Some more pious Traders act in support of missionaries, providing transport and an iron fist to back up the spiky black velvet glove of the Imperial Creed. Traders also frequently make their presence known in Imperial war zones; while most would hesitate about placing their incredibly valuable lives on the line, war profiteering is a well-established tradition in the Imperium, and traders have been known to sell arms, or offer transport services, to the Munitorum in times of crisis. As the possessors of large fleets, Rogue Traders are sometimes called on by the Imperial Navy to assist in pirate-hunting or in repelling major threats to Imperial spacelanes.
- Freebooterz, which are like Rogue Traders, but with more WAAAGH!
- Warhammer 40,000: Rogue Trader, the first edition of Warhammer 40,000.
- Rogue Trader (RPG), a role-playing game from Fantasy Flight Games where the player characters are a Rogue Trader and his or her retinue.