Queens are the ruling class of monsters.
They feed on planet's energy and resources to create a hive called a Nest where new soldier Classes are bred. Queens leech the planet until it loses the ability to maintain life (corrosion) and presumably move to other planets to continue.
High Rank monsters are created directly within the Queen's nest, making the Queens a much greater threat than a normal plant which lacks the ability. The Plants have their variety limited to their function (similar to mechanical factories) being preset to produce certain types of Beasts. Plants are born from Seeds, which can be launched through space directly to battlefields and thus can be very useful.
Oddly, the Queens shown so far are the Beasts who bear the strongest resemblance with the human physique.
The Queens so far mentioned have spanned the ranks from S then A to D in power. In the Battle of Velchees, a pair of twin queens, named E-99 and E-101 were listed as S-Ranks.
Alongside their 'production' ranking, Queens also have types. Note that the type of a Queen may refer to it's behavior pattern rather than it's power:
E prefix short for "Elise", which is stated to be the most destructive among the types of Queens, possibly also the most powerful Class, even with the exceptionally powerful E-34, who produced two Type Zeros, A and S class at that, being classified as an Elise. (Ep0 - Ch25)
D prefix for "Dile", a fairly high level, producing a Type Zero whilst it is stated that most Queens do not.
S prefix for "Sirius". Most details are pending translation.
Nests are the primary base of the Queen, where they lurk while producing High Rank Beasts and directing its swarm. Queen's Nests are like fortresses, being nearly impenetrable and surrounded by a powerful barrier.
The defenses of a Dile class Queen's Nest are so durable that Admiral Norton, even with orbital superiority, would rather use "Mantle Buster" rounds to destroy the surrounding corruption that supplies the Nest with Energy, a process that would take 5 months and end in the abandonment of Planet Void, rather than utilize his position and risk an all out assault on an active Nest.
The Queen can change the layout of the Nest to open or close off passages, as well as presumably reshape and change the properties of the barrier surrounding the Nest to respond to concentrated fire, this is called the "Nest Control Function".
The death of the commanding Queen cripples the defenses of the Nest, appearing to disable the barrier, leaving only the tough exterior of the Nest and which Beasts remain to defend the now deactivated Nest. (Prologue - 1 and 2)
In (Ep0 - Ch2) Anne invades the nest of the Queen on Planet Void through a secret route leading directly to the Queen's chamber, which couldn't be closed due to damage to the nest.
Not yet shown in series, was responsible for the hugely destructive battles 55 years before the Battle of Velchees.
S-ranked. Not shown in series, were responsible for the invasion of 7 planets in the Battle of Velchees and the production of the first pair of Twin Type Zeros.
They produced the EX-Type Zeros Cross Eye Alpha and Beta.
Appears in Ep0 - Ch1, was responsible for The Arin War.
Appear in Ep0 - Ch2. She invaded Planet Void.
Appear in Another Episode 1 - Father's Day.