Warp
The Warp, also called the Immaterium and the Realm of Chaos, is a paralled dimension of random energies and mayhem. It is the source of mortal corruption, for it is where the Chaos Gods reside and from whence their daemons come. At the same time, almost every sentient being of almost every race - including Man - has a Warp soul, and Chaos is in fact the reflection of our darker urges. It is also the source of all psyker powers, both used by those who are aligned with Chaos and those who are not. Although the Warp is one of the greatest threats to humanity, it also allows interstellar travel and communication (through humanity's psychic Navigators and Astropaths), without which the Imperium of Man could not exist.
Warp travel
Practical interstellar travel requires the use of Warp to overcome great distances and the speed of light. Starships may enter the Warp, and then emerge in real space great distances away, at a different location because space – and even time – act differently in the Warp dimension. However, there is always a degree of risk involved, as ships may be destroyed by voracious predatory Warp creatures or by storms, or emerge in a completely different place (or time) than intended.
Certain warriors also make use of the Warp for individual movement. The Eldar Aspect Warriors known as Warp Spiders use personal teleport packs that work by allowing them to travel short distances through the Warp during combat, while the teleportarium arrays on space-borne Space Marine ships allow Terminator-armored Space Marines to be teleported directly onto the battlefield. This is effective but very dangerous, especially for Eldar (because of Slaanesh's hunger), and even Terminator armor has been known to emerge from teleportation with unexplained claw and tooth marks on their armor surface. Chaos Sorcerers, Space Marine Librarians and daemons (e.g., Bloodletters) may also Warp-teleport.
Warp storms
Sometimes the Warp "aether" can become (even more) disturbed and rage in chaotic Warp storms. This is sometimes natural, and sometimes due to disturbance by the powers of Chaos. Regardless, these can severely throw off, or destroy, any starships caught in them. While some last mere weeks or months, others have been known to rage for centuries on end, and long-lasting Warp storms effectively deny interstellar travel in their area, isolating star systems. Diffuse and powerful Warp storms were a major reason for the fall of Mankind in the Age of Strife.
