A Queen is a huge Xenomorph that serves as the mother and leader of a Xenomorph hive. Her appearance differs from that of a drone, with a pair of extra arms and a large head crest. Individual Alien Queens vary in size, age, and appearance.
They can engage in combat, but usually remains stationary, letting the Praetorians (or drones and warriors, if none have developed yet), do their dirty work while they direct the hive and lays the eggs. A Queen can disconnect from its egg sack by pulling its tail out as seen in the 2010 AVP video game. A Queen is much more intelligent than a normal Xenomorph, displaying traits of anger and vengeance beyond any purely instinctual behavior. An example of this is when the Queen saw her nest destroyed by Ellen Ripley, but more importantly, Ripley killed all her children and then began a murderous rampage in an attempt to kill both Ripley and Newt, even going so far as to hide herself in the landing gear of their dropship and follow them back to the USS Sulaco. Ultimately, Ripley finished the Queen off by throwing her out the air lock.
A royal facehugger is the preferred method of developing a queen. It is a facehugger with a darker coloration who can implant a normal embryo and a royal embryo into a host. A variant of this appeared in the Aliens vs. Predator comic and appeared to be slightly darker in coloration than a normal facehugger, and had what appeared to be bristles or spines on its back. It, apparently, only contained a Royal embryo.
It is possible for a typical drone/warrior Alien to develop into a queen in the absence of either a Royal facehugger, Praetorian Facehugger, or Praetorian with the help of Royal Jelly. In a deleted scene from Alien, the alien appeared to have been turning some of its prey into eggs, perhaps showing the early stages of such a transformation, or perhaps to create a Royal facehugger that would be able to create a queen. In AVP: Requiem, the Predalien, born from a normal facehugger and capable of impregnating hosts (preferably pregnant women) directly, rather than laying eggs. It is possible that this was simply to establish a hive. More likely is the possibility that this form of "breeding" is a trait inherited from its Predator host, and may be unique to that species.
The Queen, despite her size ratio compared to other Aliens, can sustain near-limitless gunfire before having to retreat. Due to this, once outside of her larvae facility, the Queen can destroy most prey within seconds. It has also been noted that the Queen displays numerous signs of extreme muscle density, making hand-to-hand fighting a lesser-used tactic in AVP. Her height will often vary greatly, at times being roughly 15 feet and sometimes can grow to as large as 40 feet.
The Queen is the second most intelligent breed of Xenomorph and they have been shown to know how to operate machines and somewhat understand humans (actions, not speech) as the Aliens Queen "told" her warriors to back off when Ripley threatened her eggs. It should be said though that all Aliens have shown intelligence in some respects, namely observation learning but it is known that the queen is superior. In the film Aliens and the comic Aliens vs Predator: Deadliest of the Species, the Queen has shown to communicate and "bargain" with humans, in Aliens, the Queen had her warriors back off after Ripley threatens her eggs with a flamethrower, though Ripley later kills the eggs anyway after one opens, either due to the belief that the Queen did it or didn't believe a facehugger would be restrained by the queen and would attack them. In Deadliest of the Species, Caryn threatens a royal egg with a spear to get the Queen to release her friends, the Queen, however, begins squeezing them till they bleed, giving Caryn her own threat. Caryn then places the egg in front of the Queen telling her she (Caryn) will be a host to a royal facehugger if she (the Queen) lets her friends go, the Queen complies and releases the captive humans and gives Caryn a royal facehugger, which Caryn uses to facehug herself. The Queen, from that point on, protects the group though this is instinct to protect her heir than actually protecting the group because of a deal.