I think "unschooling" is a hard term to define because unschoolers consciously try to avoid too detailed a description. This used to drive me crazy, but I am thinking about it a bit differently these days because historically, "unschooling" came about as a protest to overly systemized, institutional methods.
In fact, one of the the key concerns in unschooling is to avoid too systematic a description of how it "should" work, because a specific formula will limit the possibilities of how people learn, and their emphasis is to *expand* the possibilities.
My perception from reading the unschooling literature is that unschooling is usually quite parent-directed. The parents see themselves as reclaiming the educational territory back from outside forces -- curriculum providers, government schools, "experts", etc. It is not a coincidence that many though not all unschoolers are what GK Chesterton called "distributists" --independent thinkers, proponents of voluntary simplicity, owners of their own businesses or
homesteads. Because they are often independent, strong types themselves, one of their key agendas is to impart or "disciple" that strength and individuality to their kids by expecting their kids to stand on their own two feet, own their own education, and make their own choices.
"Child-directed" has always been a frustrating term to me. To me it connotates chaos and the impulse of the moment. I prefer to think in the Ignatian terms of "consent of the will". That is, the child is brought to see that what he *wants* should coincide with *what is best* and the parent should help him with that agenda. Perhaps this is purely another way of saying the same thing. See Alicia's Blog.(this is about Fallen Nature vs Become Like Little Children)
But "unschooling" really rings with me in many ways because it implies a conscious decision on the part of the parents to focus on all aspects of learning, not just an outside-driven scope and sequence. It puts the ultimate responsibility back on the part of the parent, where it belongs (implicitly, the parents under God). The parent's responsibility is to raise the child to act and think rightly. Philosophical unschoolers see that very clearly, I believe, though secular ones may differ from me in what they see as "acting and thinking rightly".
But my main point is that it may hard to find a concise description of unschooling simply because "unschooling" resists nailing down specifics too closely -- which I think is legitimate because often over-specificity works against general principles.