[@Torack] Right, so the warp is a dimension parallel too, and fueled by our dimension (aka realspace). It's a realm fed by psykic power, emotion and the strength of souls both living and dead. It is in fact also called 'the Sea of souls', And honestly it helps to picture the warp as an ocean. The Astronomicon functions almost exactly like a lighthouse and ships 'sail' through the warp with the aid of special navigators who can read the these ethereal and sinister 'winds and waves'

A warp storm is basically a dangerously high amount of psykic energy converging within a relatively small area. These storms typically only happen in the Immaterial realm but can sometimes be so violent they tear a temporary whole into realspace. Such a tear can allow daemons to pass through into the mortal realm and even sustain themselves (Daemons can't simply stroll into realspace and stay there permanently. without a source of regular power or a mortal host they will wither and fade back into the warp realm.) as long as they maintain proximity to the storm. However the vast majority of warp storms are only temporary and will fade, and daemons that were using that storm as a well of power to remain in realspace will typically disappear along with the tear. 

Again, think of a warpstorm as a sea storm. Because that is the metaphor GW themselves often use. Warpstorms are violent, they can destroy ships caught in them, blow them way of course. They are so dangerous that it's near impossible for navigators to plot a way through them, or for the ship to fight the proverbial waves. Just as it would be similarly dangerous for a modern fishing boat to try and sail through a hurricane. Generally speaking, it will end poorly.