What defines an anarchy? It could be called an environment in which all rights are granted. When people think of such a list of rights, they normally notice immediately such rights as the right to murder or steal, however, these are not actually threats to anarchy and are balanced by the right to self defense and the right to vengeance. The one right that particularly threatens anarchy on a fundamental basis, is the right to form a government, and thus, the right to restrict rights. As such, an environment in which all people have all rights, is, by its nature a low entropy state.
So, how else can one define an anarchy? The other definition that comes up is simpler: any populace lacking a government. It is theoretically possible, that such a populace could be raised with principles designed to protect themselves and their anarchy. Thieves could be dealt with more easily without trials and courts to protect them, and murderers would be kept in line by the heavily armed population. Giant armies would emerge from out of the population to strike down threatening governments whether foreign or domestic. Such a society could generate law without government, and maintain all the trappings of civilization. However, looking at such a society one must realize that it's just a system, albeit a weird one. In such an anarchy, it is possible that anyone who doesn't conform to societal norms will be killed on the spot, or that giant witch hunts will be undertaken by a paranoid populace. IE, an anarchy CAN BE a tyranny. Most governments, (or any other system of organization of a populace.) try to justify themselves with some form of divine righteousness. Once you realize that an anarchy can be tailored to build any societal model, it becomes a system like any other, acceptable only upon the merits of its abilities.
So, given that you make an anarchy, what is different from another government, the same in respects separate from systematical factors? First off, in an anarchy, activities must be popular- basically, passing a "bill" in an anarchy, is equivalent to passing a constitutional amendment in a democracy. Such changes to "law" would have no official ceremony- the nation would just evolve over time. What's most important here is- war. An anarchy would be capable of starting a war, only after a rather extreme event managed to raise extreme support. The nature of this event, is, dynamic, in that it depends on what the populace thinks of as an act of war. In this way, anarchy resembles democracy, but requires generally bigger events than an equivalent democracy. However, just starting a war isn't enough in anarchy. After all, in an anarchy, the army can, literally, just pack up and leave at any time. As such, for an anarchy to engage in war, the war must be popular throughout. This leads to a fork in how an anarchy operates-
In a military anarchy, an army would always exist, and invading and conquering foreign nations would be seen as the norm. Alternatively the anarchy could form a army only when invaded, defending itself from foreigners but rarely fighting frivolously. In-between systems are too complicated, and thus, difficult to manage in an anarchy.
One should not make the mistake that an anarchy would not have leaders. In any system, certain people stand out, and are listened to. While anarchist leaders would not dare judge criminals directly, or issue ultimatums (for fear of a anti-government crowd labeling them a start up government, and executing them on the spot.) national policy would be very dependent on these people. Given that there is no system of turnover, it is likely that these people would be very established, and would wield their power for life (though many would fade in and out of the spotlight in the flow of time.)
To maintain an anarchy, certain factors are necessary, chief amongst them- homogeneity. Amongst multiple cultures, a clash is inevitable, as well as a power struggle. This power struggle is almost certain to include a government. Therefore, either the anarchists must ethnic cleanse the opposition, or the nation ceases to be an anarchy. What is most interesting, is that an anarchy must thence have strict border control, especially assuming its neighbors are all living under, and supporters of, governments. Even anarchist neighbors could be living under a vastly different anarchy, and would therefore, start enforcing vastly different laws. Given that in an anarchy just living in the country makes you a full citizen capable of taking the laws into your own hand, it is imperative that immigration be controlled, lest your country dissipate.
