The Spieces 8472 is the name that the Borg call a race of Insectoids from a parallel universe. Not being satisfied with fighting and conquering races in our universe, the Borg attempt to control another universe. The other universe is known as fluid space because instead of nothing, there's fluid. Not only do Species 8472 have to deal with the Borg, they also have to deal with the Hirogen who are hunting them.
Due to the a pact between the Borg and Voyager, Species 8472 targets Voyager with attacks.
In the Voyager episode In the Flesh, the Species 8472 have created a space station in the middle of nowhere. The occupants of the space station are human in every way. The humans are in fact Species 8472 who are using it as a training ground to attack humans. Captain Kathryn Janeway manages to call a truce between humans and Species 8472 and swap information that is of use to one another.
Species 8472's real name is revealed to be the Undine in Star Trek Online.