The assault swarms of the Tyranids are made of a multitude of creatures, seemingly spawned from mankind's darkest nightmares. In wave after wave, clawed and fanged monstrosities scuttle, leap and slither across the battlefield, intent only on the destruction of their foes. With scything claws and razor-sharp teeth, the Tyranid broods rip, tear, slash, maim and mutilate everything in their path, their killing frenzy leaving nothing but dismembered corpses in their wake. Tyranid attacks are characterised by large broods of creatures sweeping forwards to engage the enemy, whilst the larger bio-constructs work into the best position for their devastating ranged attacks.

-from the Warhammer 40,000 rule book

Just believe me, these guys are very frightening! The only hope you have against them is to shoot them all before they get to you. Once they get into hand to hand combat, there is no stopping them.