Tyranids are a xenos race that come from outside of the galaxy. They are animalistic monstrosities that are connected by a hive mind, and their sole purpose is to consume the galaxy and move on to the next.
Genestealers are the vanguard of the Tyranids, using their ability to infect a host species in order to seize control of planets ahead of an invasion.
Tyranids first entered the galaxy in the 41st millennium, where their presence was first discovered by Inquisitor Kryptman on the planet Tyran, though they had already stripped the planet barren long after he arrived. The Tyranids consumed every ounce of biomass on the planet to grow their hive fleet. He quickly realized the danger that these new Xenos posed and set to work on finding countermeasures against them.
Tyranids are all connected by the Hivemind, the collective consciousness of their race. Each individual creature is connected via synapse creatures, who act as nodes for the smaller and more numerous tyranids. Some Tyranids are intelligent enough to operate away from the main hive fleets and act as vanguard creatures, such as genestealers and lictors, or else act as leaders, such as the Swarmlord.
Tyranids are 100% bio-organic and lack any inorganic technology. And though they cannot use he Warp for faster-than-light travel, their mere psychic presence completely disrupts the Warp and leaves any world they invade unable to call for help. They are also constantly evolving, using the biomass of the organisms they consume to develop new defenses; as such, they are completely immune to disease and poisons. However, a mutagenic acid was discovered that is quite effective against them.
At least for now, anyway.