- We are the Borg. You will be assimilated. Resistance is futile. -- The voice of the Borg (VOY: "Scorpion, Part I")
Assimilation, in its late 24th century context, is the process whereby the Borg Collective adds new beings and technology into itself. In accordance with their goal of attaining "perfection," the Borg often assimilate entire species, leaving behind dead worlds and few or no survivors.
In addition to the physical mutilation that occurs, being assimilated as a Borg drone robs a person of their individuality, enslaving them to the will of the Collective. Most sentient beings consider assimilation by the Borg to be a fate worse than death.
It is extremely rare for an assimilated being to be freed from the Collective, though it has happened on several occasions. The two most famous cases of reversed assimilation known to the Federation are Jean-Luc Picard and Annika Hansen.
Elizabeth Singh, after surviving the Borg attack on the USS Rebellious (NCC-1807) at the Battle of Wolf 359, witnessed her twin children, Esthappen and Rahel, being assimilated by the Borg. (Star Trek: The Cantabrian Expeditions: "Devil's Cube")