Anarchy, in it’s purest sense, is to a system what darkness is to light. Darkness is the absence of light, not a thing in-and-of itself. Anarchy is the lack of an establishment or system, rather than a system in itself.
What this means, in practical application, is that most anarchists are simply opposed to whatever system exists currently. Human nature dictates that SOME system will exist as long as we do, so true anarchy can only exist when there are no longer humans around to perceive it.
In historical context, this almost always means that when anarchy “takes over,” what it creates is a “systemic void” which - like any vacuum - quickly gets filled. Usually by the guy with the biggest stick.