The hive is the primary structure of the zerg and is the key that unlocks the final stages of their evolution. Protected by a dense exoskeleton and granted full access to the Overmind's pool of knowledge, it continues to provide the same functionality that it has since it was a hatchery (birthing larvae, maintaining creep, and digesting minerals/gas). Once the hive is present, the being commanding it has the potential to unleash the full fury of the zerg.
The genetic model for a hive is extremely complex, and would be indecipherable by any terran scientist.
Many zerg ground units may burrow, become invisible and may be revealed primarily by detectors, and may not move or attack. Exceptions are the roach, which may move while burrowed, the infestor, which may move and cast Infested Terran, and the baneling, which can detonate while burrowed.
Some units can unburrow via autocast, causing them to emerge and attack when an enemy moves into range.
Queens can target a hatchery/lair/hive, causing it to eventually spawn four larvae. These join any already-present larvae. The larva-producing structure will not naturally produce any more larvae until the total falls below three. The larva count for any given hatchery/lair/hive cannot exceed nineteen, no matter how many times spawn larva is cast.
The hive for StarCraft II was based off the lair in StarCraft, in order to make it look more 'evil'. There were also plans to change the middle maw portion more gradually as it evolved from a hatchery, but there wasn't enough time to do so.
↑"for zerg is it the same that the hatchery sets the rally, or can you rally for each individual egg?"
You can do both. You can set a rally for your hatchery, but can then override it with manual orders per egg as well. Also don't forget, you can set a worker rally point for your hatchery, as well as a unit rally point. Karune, Rancid_death75. Another rally question for Kaurne!Battle.net StarCraft II General Discussion Forum. Accessed 2009-02-18.
↑Blizzard Entertainment Staff. 2010-07-24. Infestor. Blizzard Entertainment. Accessed 2010-07-24.