"The Hive is the focal point of the Swarm, the pinnacle of the species' millennia-spanning evolution. For zerg units, the Hive is home...and nothing says "home" like bubbling lava and black volcanic rock."
Protected by a dense exoskeleton and granted full access to the Overmind's pool of knowledge, the hive 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. When under attack, hives can harden their hull with bony protrusions, allowing it to shrug off weaker strikes.
During the Great War and Brood War, the Zerg Swarm could deploy a "Char hive." This hive glowed by the perpetual heat it generated, and while it was not so hot that melee attackers would be damaged, the drones and larvae within it were able to take advantage of the warmth it provided on the colder planets of the Koprulu sector.
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.
Kerrigan, Zagara and Abathur use the hive as the tier 3 or final upgrade of their hatchery. It retains the previous functions of the lair to create larvae, generate creep to cover the nearby area, and finally train queens. Kerrigan and Zagara can create regular queens after morphing a spawning pool. Abathur's hive can initially create swarm queens instead of regular queens, and can be upgraded to create two swarm queens simultaneously.
Note that while Abathur retains the standard Infestation Pit pre-requisite, Kerrigan and Zagara are free to morph the hive immediately after the lair finishes. If there is sufficient gas to do so, this allows those two commanders to access the higher tier units or upgrades quicker than would normally be expected.
Dehaka use the primal hive as a primary structure which can breed primal drones, remotely create extractors on vespene gas geysers, evolve attack and carapace on primal zerg units, and it can be uprooted to move and also attack.
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.