The Tyranids are an extragalactic sentient ecosystem of innumerable different species based on the same genetic material dedicated, under a unique hive mind called the Synapse, to survival, propagation and evolutionary advancement originating from an extragalactic from outside the Local Group. They travel across the universe at supraluminal speed in their fleets of gigantic hive ships, systematically consuming all biomatter they meet. All Tyranid organisms in a fleet are synaptic and contribute to a communal Hive Mind.
The components of a Tyranid Hive Fleet travel almost exclusively in large groups known as swarms that possess specialized biomechanical creatures for destroying and consuming a wide variety of prey life forms. Through their invasions, Tyranids acquire new biological traits from other lifeforms, processing these new traits through their reporductive process managed bhy their Norn-Queens. The Tyranids do not build but use gene-splicing and bioengineering to grow what they need from the organic biomatter harvested from the planets they consume.
It is believed that tyranid advanced parties will scout the Milky Way around the 35th millenium leading several hives behind them around the 41st millenium from different directions.
Source: Warhammer 40,000 universe.