Alright my turn.
THE WARP
The warp is another dimension that is strongly linked to the material realm, effectively everything that happens in the material realm is echoed inside this would-be void, where no matter exists. It is made up of the psychic footprint of everything in the material universe and consists of raw emotion as everything, living or not, has at least a tiny soul. And because of this it is home to the
Chaos Daemons.
It looks something like this:
Without the warp, nothing would be possible. The Imperium is so vast, spanning the whole galaxy, that the warp is necessary for travel, psychic communication via Astropaths (human warp vessels) and defence. Yet the warp is also the realm of Chaos, where all the negative emotion and psychic feedback coalesces into the form of four great powers of Chaos, and when a psyker becomes active in the material world, it creates a possible gateway for daemons to emerge from and tear into the real world.
Warp travel:
By using warp engines, Imperial ships can make faster-than-light jumps through space, and so the galaxy can be crossed in a matter of months. However, doing so unprotected would mean the ship would be devoured instantly, sucked into the warp, becoming possessed and soon floating back out as hulk, infested with daemonic corruption. So Imperial ships travel through the warp with Gellar fields to shield themselves from these forces.
The warp is probably the most dangerous aspect of human existence, as constantly new psykers are born who have no training or shielding, and are easily tempted by the voices in their head to embrace their power and unleash a daemonic horde. So the Inquisition conducts an operation to round up these psykers and... dispose of them, depending on their strength.
Instead of magic, 40k has psychic power. This is directly harnessed warp energy that when controlled and wielded by a strong mind can bring devastation or provide foresight. Ork psykers manifest power in destructive, clumsy ways that increase in volume the more Orks are around. Although everything has a soul, all Eldar (excluding Dark Eldar) are vaguely psychic, often represented by a telepathy in family or close kin, and Eldar make the best telepaths and diviners. Humans are said to be the most powerful psykers of any species when trained properly, which takes time and effort and rarely amounts to many succeeding, but when they do they become capable of manifesting almost anything. Other species like Tau have little knowledge of the warp and barely register, whereas the Necrons despise the warp, waging war against it whenever possible.
And then you have the four Chaos Gods who live in the warp, who only exist because they are made up of the emotional discharge. Ironically Chaos needs the continuation of mankind and others in order to keep existing, as without the psychic feedback they would simply dissipate. There are also other aspects such as the Old Ones, who first opened the realm of Chaos to fight the C'Tan, and the Eye of Terror, btu I've rambled enough.
I can't believe I bothered writing this.