Fractal - n. "In colloquial usage, it denotes a shape that is recursively constructed or self-similar, that is, a shape that appears similar at all scales of magnification and is therefore often referred to as 'infinitely complex.'" (Wikipedia)The Borg collective exemplifies this colloquial definition of a fractal - the idea of something that is the same at all levels of magnification. In effect, the Borg are a fractal civilization.
The Borg, on the largest scale, form a collective. In fact, Seven of Nine often referred to the Borg simply as "the Collective." We can mentally picture this - swarms of cubes populated by a vast array of interconnected drones all fulfilling specific roles within the collective. Tasks are carried out for the whole. As much as each drone may perform some particular task, this specialization is part of a larger scale decentralization. There is redundancy built into the very fabric of the collective. Eliminate a drone, damage a portion of a cube, and the collective continues to function and will repair itself. This description of the collective as a whole is also true on smaller levels of magnification.
Although Borg ships have been most frequently observed by themselves, it is clear that within the vast collective there are at times smaller "collectives" of Borg cubes (and spheres) that work and operate together. For example, the fourteen cubes Voyager encounters at the beginning of the episode "Scorpion", or the Borg ships seen surrounding the unicomplex in "Dark Frontier". The very nature of the hive mind suggests that such ships are not operating independently, but function together as whole. Of course, the destruction of an individual cube does not impede the progress of this miniature collective.
Increase the magnification by another factor and we see that each ship is a collective unto itself. Each drone, and each part of the ship for that matter, fills a role in concert with the whole. And yet the collective on board a given ship is further divided into a myriad of sub-collectives. The very name Seven of Nine demonstrates this structure - a small group of nine drones. Each group functions as a unit, forming a collective with all of the other groups. And yet another collective is nested inside each group as each drone carries out a role within this small, immediate collective.
finally, each drone is a collective. All of its nanoprobes work in concert with one another carrying out the collective will of that drone - to the extent that it relates to the will of the larger collective. It stands to reason that even these nanoprobes are divided into sub-collectives devoted to particular tasks, each one in constant communication with those in its group, and each group coordinating with the other groups. The nanoprobes function like a miniature collective of Borg cubes, but instead of swarming through space, they swarm through the biological tissues of individual drones.
When a person is assimilated, he immediately becomes part of the collective. The nanoprobes themselves function as microcosm of the collective as whole. They imprint the very essence of a collective into the persons blood-stream. Once the nanoprobes are in place, the person becomes embedded in each layer, in each iteration of the fractal whole - The Collective.
Intimations of the Pattern - Whether by accident or by design, the fractal nature of the Borg is reflected elsewhere within their civilization. Borg "script" resembles a fractal pattern. In fact, I wouldn't be surprised if the branching green patterns were generated by code similar to the code for fractal tree and vine simulations. Also, the Transwarp hub, featured in the Voyager episode "Endgame", has a fractal look to it - repeating patterns at smaller and smaller scales. (See the Transwarp Hub over at Ex Astris Scientia's gallery of Borg Ship Classes.)
On the Matter of Unimatricies - The term Unimatrix has been thrown around as a unit or division within the collective. Each unimatrix is a miniature collective, much as I have described above. We know that the Unicomplex is called Unimatrix 001. Beyond that, I am not certain if the term has been used consistently enough to be precisely defined. Is each Borg ship a Unimatrix, or does a Unimatrix encompass multiple ships? In other words, where does a Unimatrix fit in the layers of collectives? Any thoughts or observations on this would be appreciated.