From Exterminatus Manual
The Tyranids are a race of biological predators that evolved outside our own galaxy and have travelled through the void to reach us. Although classed as one race they are in fact made of many different species effectively slaved by a Hive Mind that acts through the race as a whole.
They posses purely organic technology, adapting and adding to their capacities by the consumption and assimilation of others genetic code in order to advance there species.
To do this they consume entire worlds, wiping all biological life on them out leaving only non-organic materials behind. The Tyranids are a parasite, consuming everything in its path and moving on, in this way it is theorised that there original home galaxy was wiped clean of all other life and thus they have had to search out new sources of genetic material.
A fragment of one of the largest hive fleets, codenamed by the Imperium as "Behemoth" is featured in Exterminatus, they can be recognised by their black bodies and crimson carapace.
The lower left flood chamber shows how much health you have remaining and the lower right flood chamber shows how much bio-ammunition you have. Bio-ammunition regenerates on its own.
Genstealers are the shock troops of the hive fleet, very fast with sharp, rending talons. Their reletively small size an leap ability enable them to ambush the enemy and strike behind their lines.
Tyranid Warriors are the closest thing to a "basic" synapse creature in the ever adapting and ever-mutating forces of the Hive Mind. Very capable at both long and short ranged combat with their living weapons, a Deathspitter and Talons combination.
Lictors are the stealth and reconnaissance units of the hive fleet. They are the eyes, ears and sensory organs of the hive mind behind enemy lines. Solitary and silent their cloaking ability allows them to get into striking ranges of their talons.
Zoanthropes are the psychic artillery of the tyranid army. By directing the awesome power of the hive mind to intervene in the physical universe they can swat whole groups of soldiers in a single strike and blast vehicles apart with a mere thought. So focused is their mental development that their limbs have shrivelled to mere stubs. Zoanthropes use a form of telekinesis to hover silently about the battlefield.
The biovore has an artillery like role, deploying spore mines over great distances to deny access to areas or paths to the enemy. Spore mines are large, slow moving, inaccurate projectiles that cause a moderate amount of damage over a large area when detonated. The biovore's slow speed and poor melee attack are are it's major weaknesses.
Carnifexes are living engines of destruction. With a thick carapace that can soak up damage, talons that can rend through armor and a lethal plasma scream a carnifex is the pinnacle of the Tyranid classes.