Species 8472 is a race of telepathic creatures. Since no one found out their races name, everyone calls them by the designation the Borg gave them, Species 8472. They live in fluidic space, and have very good immune systems. Their bodies are not affected by the Borg assimilation process. In a matter of fact, the Borg can't even touch them without taking serious damage. Their ships are very powerful, and they can destroy even a Borg Cube with a few shots.
After the Borg tryed to assmilate their species, and failed, the came back again, this time infiltrating themselves so they look exactly like Starfleet Officers. They considered the Federation as a threat because they found a way to destroy these bioships. Eventually, after talking, the crew of the USS Voyager, solved the problems, and the two races seem to be in peace now. They are a very cunning and adaptive species, and they should be taken seriously. They could still be a threat, however, to anyone who doesn't know how to fight them.