To the Borg, individual thought and action are considered imperfect and weak. They travel through the universe in their hive cubicles seeking new civilizations to assimilate into their own consciousness, or in their words, "add the biological and technological distinctiveness of to their own". They don't simply assimilate individuals, they devour cultures, races, entire worlds in achieving their understanding of perfection. If a Borg drone dies or is beyond repair, his memory will still be retained by the collective. So in a sense all Borg are immortal.
Each Borg has cybernetic implants that differ with the specific designated task. The assimilated drone has only to listen to the murmur of thousands that transmit into his mind precisely the task at hand, never needing to decide independently nor worry about what course of action to take. The result is an outward harmony and complete cooperation among the drones at the price of surrendering individual thought. Yet such a life of security has prompted Borgs separated from the collective to long to lose themselves within the collective again and return to the hive.
Assimilation is the Borg's means of adding or replacing drones, as well as their sole purpose of existence. New technology and knowledge is Borg symbolgathered in this way, and nearly every species encountered has been assimilated easily. Species 8427 is the notable exception. Assimilation is achieved by injecting nano-probes below the the victim's subcutaneous layer into the bloodstream, whose blood cells are then attacked and subjugated by the probes to work together and lay the groundwork for connection to the collective. Thousands of such probes can be injected within seconds, making the effects of initial assimilation immediate. Bio-technological interface with Borg cybernetic implants and attachments are also created by the nano-probes in preparation for surgical procedures that may include modified extremities for specific tasks, or optical and aural implants. Constant to all assimilated drones is the neural implant that is unremovable once installed. It is the actual connection the drone has to the rest of the collective, relaying all orders while maintaining records, coordinates, and other information about the drone's activities.
Hive Structure
The immense knowledge of all the assimilated Borg minds function as one, and each Borg is part of a giant subspace communications network called the Borg Collective. All critical information is relayed to each Borg through this network, allowing the instantaneous adaptation and reaction to any outside factors. The Collective is organized into a social hierarchy composed of several social hives with drones and a Borg Queen. The Queen does not function as a breeder of new drones, but the central consciousness that dictates the will and motives of the collective. She uses the pronoun "I" instead of "We" but does not appear to be truly individual, instead maintaining an awareness above that of an ordinary Borg. TNG "Q Who?" 2-42, MOV First Contact 8, VOY "Scorpion, Part I" 3-68