The Borg in Star Trek are the most numerous species of the Milky Way with no less than 50 000 billion individuals. It is actually a group of several races. They have their own territory that is very extensive, ranging from Delta Quadrant to Quadrant Beta, forming the largest known colony.
Their only goal is to assimilate a maximum of species to achieve perfection. A single individual is enough to convert billions. They can travel across dimensions.
Because of this, they sow chaos after their passage. Their biggest obstacle was species 8472 whose genome was too complex. 8472 is the only race capable of destroying the Borg.
Whenever the Borg encounters a species, they give them a number.
They are cyborgs. They are all connected together. And must meet via the Collective to make major decisions.
The Federation can transform a Borg into its original state, ie before being assimilated, but often with great difficulty. The Borg nanorobots responsible for transmutation are very tough.
Every individual is a drone. Their vessels are cubes (sometimes spheres or pyramids), they are undoubtedly the most powerful of the galaxy.
A part of the Collective is dissenting, it would like to change things. This is the Unimatrix Zero.
A few questions arise. If they are so numerous, why do they not assimilate the whole galaxy rather than conquer other dimensions? Is it to better preserve their territory ? Their behavior seems erratic.