Species 8472 are undoubtedly one of the most unusual organic lifeforms ever encountered by the Federation. Their origins lie in 'fluidic space' - a higher dimension which is apparently composed liquid rather than the empty space of our own universe. 8472 are the only species in their dimension, and had no conception of other lifeforms until the Borg invaded their territory. Unfortunately - for the Borg - species 8472 have an incredibly efficient immune system; any form of biological or technological intrusion into their tissue is instantly rejected and destroyed. This renders them immune to Borg nanoprobes, and hence incapable of assimilation. In addition, their dense structure is highly resistant to many forms of energy such as transporters and weapons.
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Physically, species 8472 has a roughly humanoid head and torso on a tripedal base. There is a mouth but they do not usually communicate by speech, instead relying on telepathy. The extent of their telepathic abilities are unknown, but are thought to be very long ranged, perhaps even trans-dimensional. Beyond this little is known about this enigmatic species - few have survived a close encounter long enough to gather comprehensive data!
Even less is known about the culture and philosophy of species 8472. In the one known telepathic contact with them the message 'the weak shall perish' was transmitted; although worrying given their numbers and level of technology, the outlook of a whole species can hardly be judged from one contact with one individual. 
A more balanced view was possible when Voyager encountered a large space station manned by Species 8472 and containing a replica of Starfleet academy. Voyager investigated this station and discovered that it was being used to train members of the species to infiltrate the Federation. Ultimately the Voyager crew was able to come to an accommodation with the leaders of the station, establishing a state of relative peace between the two sides.