Here is the latest (as of posting) edition of the Tyranids codex for the tabletop game Warhammer 40,000. This full color cover to cover scan includes everything you need to play an army of deadly fast, pointy, and crunchy bugs in the 40k universe.
Ever seen the movie Starship Troopers? Yeah, i know the movie sucked, especially compared to the book, but the battle scenes therein probably have the best representation I can give you of the idea of a Tyranid army. Huge swarms of smaller, less intelligent creatures that are basically nothing more than a walking mouth, followed by much much larger monstrosities that broadcast a node based Hive intelligence. Oh, and they can also throw tanks bigger than Humvees like a baseball.
Unchecked, unstoppable, unexplainable alien aggression. Out for one purpose: to feed. These are the Tyranids.
This is an army book for one single faction in the Warhammer 40,000 universe. The main rulebook will be needed to understand these books for newcomers. I also have a torrent of this. Current edition is 5th Edition.