Piracy in space has long been a theme in science fiction, and a combination of factors makes certain forms of piracy quite plausible in the Aliens universe. The most important of these are the isolation of merchant ships in transit, the unethical nature of the corporations, and the vast unexplored regions between settlements.
Firstly, the physics of space travel and FTL. Pirates generally divide a ships journey into three stages; orbit, system travel, and stellar travel. The stellar travel stage is the FTL part of the journey, and piracy is impossible during this time because nobody can yet affect a ship going FTL. Piracy is also generally impossible during the orbital stage of a trip because planetary orbits are closely patrolled, although this problem is not an insurmountable one. A few daring raids have occured over colony worlds without permanent defense ships.
Such bold (foolish?) attacks aside however, virtually all piracy occures during the system travel portion of a ship journey. Hyperdrive cannot operate within a gravity well over a certain strength, which requires that a ship travel a certain distance out of a system before going FTL, and also forcing them to exit FTL some distance from the target planet. During these sublight trips a ship is vulnerable. Furthermore to minimize the fuel costs, ships usually travel along certain predicatable courses. Failing that, pirates can often learn of flight plans from a variety of sources, including bribery, computer hacking, or getting a tip from a rival government or corporation.
A typical pirate operation involves an intercept course that crosses the targets flight path far enough away that help cannot arrive in time, but too close for the target to escape into FTL. Getting the relative velocities right for this kind of operation is complicated and often fuel expensive, so pirates need good navigators. To make things more difficult merchants often deliberately vary from their predicted course for a variety of reasons, not least of which is to avoid pirates.
If things are going to plan, the pirate vessel will approach the target either undetected, or with jamming to prevent distress calls. A surrender demand, and possibly warning shots, will be given. If the target surrenders then the pirates board, disable communications, and steal the cargo. Most pirates leave the ship's crew alive to limp back to base, although some do kill the crew and steal the ship. If the target doesn't surrender, then the pirates will usually fight, although not to the death. Piracy is about profit, and once it because too expensive to stay they will leave. Pirates usually shoot to disable, so as to damage the cargo as little as possible.
If the target ship manages to get off a distress call, then the pirates may have to flee, but usually not. Starships travelling out prior to hyperspace can be days away from help, so unless the pirates are very unlucky help will arrive too late to be of immediate assistance. Pirate vessels are usually very fast, both at sublight and FTL, which means that only fast ships such as corvettes and fighters can hope to hunt them down. Fighter escorts are often the most successful deterent to piracy; they have the speed and firepower to make piracy unprofitable, at a minimum cost.
Countering piracy is difficult. As mentioned help cannot usually arrive in time, so the only effective protection is either armed merchant ships (combined with incentives for captains to fight for their cargoes) or military escort. If one wishes to catch a pirate, one must either set a trap (made difficult by the high speed of pirate vessels) or attempt to track down their base, which is invariably well hidden.
Target cargoes for piracy are usually small and valuable, since massive cargos leave the pirate ship slow and vulnerable. Disposing of loot isn't usually a problem since the corporations (and some colonies and governments) don't ask questions when one offers them a product at a good price. Since most pirate operations are resolved without even an exchange of fire, pirates often have plenty of spare cash. Its just those few operations that do result in combat that get expensive, (in munitions, repairs, and lost plunder) so many pirates avoid combat altogether, fleeing if the target doesn't surrender.
The slow speed of FTL communication, the scattered nature of human exploration, and the animosities of rival governments towards each other all make it easy for pirates to set up secret bases on moons or at Lagrange points in uninhabited star systems. Such operations are expensive, so pirate groups often share bases in informal alliances. These alliances don't get too large for fear of discovery, but one pirate group will usually offer repairs to another at only slightly above market rates. There are even a few repair bases that exist purely for the pirate trade, although these tend to either have a legitimate front or charge a premium for their services.
Most pirates attempt to keep on the good side of at least one corporation. Companies are excellent sources of information, fences of stolen goods, and suppliers of military hardware, all of which are essential to successful piracy. However this sort of arrangement is distinct from privateering in that neither side will confess to the arrangement, and that the corporations don't actively sponsor the pirates. To them the pirates are customers, plain and simple.
A typical pirate vessel is an outdated military vessel, although some pirates prefer modified civilian ships for the inconspicuous appearance. In all cases they will be armed, usually favouring light energy weapons over expensive and destructive missiles. Speed and fuel are the key elements of any pirate vessel. A good linear acceleration is necessary to intercept a target and escape, while fuel is needed to maintain this speed longer than either the target or any possible pursuers. A good FTL speed is very useful, but not absolutely necessary. Some pirates have operated using sublight ship and an FTL capable carrier, although this is at a much smaller scale than similar military craft.
However their skills aside, modern pirates have a very similar society to the Caribbean raiders of five hundred years ago. While they exist basically in anarchy, pirates have an uncanny degree of loyalty to each other. Piracy is about profit, and in the long term there is usually little profit in cheating another pirate, or even another pirate group. Other pirate ships are too fast, too well armed, and too unpredictable in their course to make good targets, and there is nothing to gain by selling them out to the law.
Pirate crews are commanded by the captain and the quartermaster, and which has authority depends on the circumstances. During battle the captains authority is absolute, and the quartermaster only advices on how to maximise the profit for the venture. At other times the quartermaster has the final word, the captain merely offering tactical advice. In most cases the quartermaster is the owner of the ship, the captain hired for tactical skills, but occasionally the situation is reversed. Regardless, both the quartermaster and the captain get a higher share of loot than the crew (5 - 20 shares is typical) while many vessels also have a group of officers each getting double shares. Pirate crews are not democratic, but the opinions of the crew are usually listened too (they have as much at stake as anyone else), and there is usually a formal process for resolving differences between shipmates. This isn't usually a duel, although modern medicine has made a duel to first blood a minor matter.