IMO, unschooling will not work for all families. To unschool successfully, a family should be loving, responsive, creative, and model a love of learning. Perhaps even with such families unschooling will not work for all children. My own child is driven and loves to learn, and is particularly adamant about directing her own learning. Of course, she is not-quite-four, but since it's a trait she's had since infancy I don't think it's going to disappear anytime soon.
So, the basic principle is that children teach themselves from infancy without formal or informal curriculum. Children who are loved and played with teach themselves how to walk and talk and feed themselves. These things seems simple, but they are truly amazing feats. Unschooling is the idea that kids will continue to learn everything they need to know all of their own initiative. Of course, they typically ask their friends or relatives for help in learning. Of course, they often choose to take classes. But as an unschooling parent your responsibility is to trust that your child can make those decisions and will make them appropriately.
As for the basics, I can testify that unschooled children can certainly teach themselves the essentials. My DD is not yet 4, and has taught herself to read and do basic math and is currently working on handwriting (all on her own, though I help her when she asks).
