In essence unschooling is child-directed education. Some unschooling looks nothing at all like traditional education or structured homeschooling. It might look like a string of Saturdays, for years on end. On the other hand, if the child likes daily structure, enjoys using structured resources, or enjoys being "taught" by a parent or mentor, it might not look all that different from parent-led or curriculum-centred homeschooling. What is important is not whether the child uses curriculum or not, but why the child is doing what he's doing. In unschooling the answer is never "because that's what he's expected to do" or "because his parents make him" or "because that's on the ___th grade curriculum program." The reason why is "because he wants to."
Unschooling ends up being very much a way of life for most families. Typically learning becomes indistinguishable from life. Parents and children often learn together, by doing, by following their passions or getting caught up in some worthy pursuit. Subject-area distinctions are of limited use in unschooling. Evaluation of learning is rarely a part of unschooling.
At the centre of unschooling is a basic philosophy of trust -- trust that a child, driven by curiosity and the desire to understand his world and become a competent individual capable of contributing meaningfully to that world, will learn what is necessary. That if a child lives a life meaningfully involved in family and community, with adults and other role models and resources available, competence and interest in all that is necessary (and likely much much more) will come in good time. Also at the centre of unschooling is a belief that children who are not coerced to learn will retain their curiosity and intrinsic motivation at high levels.