The Tyranids are an extragalactic composite species of hideous, insectoid xenos. They actually comprise an entire space-faring ecosystem comprised of innumerable different bioforms which are all variations on the same genetic theme.
The Tyranids are unlike any other intelligent species encountered by Humanity. They are the ultimate predators; to them, all living things, from the lowliest insect to the most advanced starfaring civilisation, are mere prey. Only now are the inhabitants of the galaxy realising the scale of the threat; unless the Tyranids can be stopped, it will mean nothing less than the extinction of all life.