One of the most deadly alien races in the galaxy, the Borg are not individuals. They operate as a single mind, a collective. Their goal is to achieve perfection by assimilating alien species into their own and thereby gaining their knowledge.
The first encounter with the Borg came when Q hurled the Enterprise across the galaxy to show them they have never faced real enemies before. They have had other encounters including in the movie First Contact and numerous contact with Voyager.
The Borg, although a single mind, are led by the Borg queen. They have trans-warp conduits to allow them to travel across the galaxy. Their ships are typically either huge cubes or smaller spheres.
There are only 2 Borg who have ever truly become individuals again. The first was Hue who was captured by the Enterprise and then released back into the collective. The second is Seven of Nine from Voyager who became a member of the Voyager crew.