From the website:
The Tyranids are unlike any other race to be encountered by Humanity. They are the ultimate predators; to them all living things, from the lowliest insect to the most advanced 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.
Codex: Tyranids contains a wealth of background and rules – the definitive book for Tyranids collectors.