The Spirit World is one of several transmundane "realities" or "layers" that overlap with the physical world. Anything done to things in the Spirit World will happen in the real world, with the world working out how to do so through contrived coincidence.
Blake stepped into the spirit world at one point but thanks to his connections he didn't fall into the abyss.
Breaking the spirirt world counterpart of and object can prohibit its use by an other but will likely lead to the destruction of a real world counterpart. This likely causes some form of bad Karma.
Has some abstract relation with reality. Finder/Chaos Mages commonly work with the spirit world.