Wikipedia gets unschooling just about right, I would say, though many home educators in the UK call a similar sort of thing "autonomous education", probably so as to define more precisely exactly what it is we do and so as not to see it as if in reaction to something else.
There is a key insight that informs the whole thing for me - one that is derived from Taking Children Seriously. It is that people think less clearly about something when they are not fully engaged with the idea. This is actually a debasement and paraphrase of the TCS expression, the full idea being being couched as a definition of coercion, ie: that coercion may be defined as being forced to enact a theory that is not active in the mind, which reduces the possiblity of rationality and creativity.
I have wrestled lengthily with this idea. Is it just a logical but meaningless formulation? Could it be the case that, (as an Etonian beak once memorably put it,) coercion means that you won't consciously remember most of what you are forcibly taught in schools, but somewhere at the back of your mind you will retain a bundle of stuff that will actually serve you well, with the resulting implication that coerced learning is therefore a genuinely worthwhile activity?
The only way I feel I have confidence in the TCS assertion is to put it to the test of my own personal experience. Yes there is stuff that is in there that sneaked in round the back. eg: I learnt Latin for some seven years or so, and recently struggled on page 4 of Ecce Romane Book One. But all that forced learning has left me with at least some ability to deconstruct words, and to recall vaguely the etymological roots of some of the English language. I also probably have at least a loose grasp of basic grammar to show for all that very painful lack of effort, and on the evidence so far, an extraordinarily meaningless A grade O level.
But the learning that matters, that resonates, that genuinely gives space for thought and creativity, for excitement and development has been when the mind is allowed to be freely engaged. So I'd say unschoolers, TCSers and Albert Einstein were right in this regard;
"Everything that is really great and inspiring is created by the individual who can labor in freedom." Albert Einstein, 'Out of My Later Years,' 1950