Entropy's description is the basic standard of literary criticism, often called structuralism--you assume that the author's thoughts and feelings are important to the text, and try to determine through the text what those thoughts and feelings are. In other words, you assume that the book has a message or theme and try to figure out what it is. All other forms of literary criticism tend to use Structuralism as a base, but they don't have to; in general, the more popular forms of modern criticism ignore the author's intent and look instead at what the book itself is saying, whether the author intended it or not.
Deconstruction is a little different because it is most commonly used in two very weird ways. The first is just what MoD described: college students who use it as an excuse to disagree with other people and accepted literary conventions. The second is almost completely backwards of traditional criticism: instead of using a form of analysis to say something about a book, people use a given book to say something about their preferred form of analysis. I used both methods pretty often in college (I was a deconstructionist almost exclusively), but I've grown tired of them, to be honest, because they smack of people trying to look smart by showing off.
Every now and then, however, you can find a piece of criticism that actually takes deconstructionist ideas and does something interesting with them. The "relying on what your denying" aspect is one of these, because it challenges the world of literary analysis in (ironically) constructive ways, forcing you to consider ideas and patterns that previous schools of thought have ignored.
In my opinion, however, most deconstructionists are too enamored with their own esoteric-ness to be of much use. The ideas need to become more common and, subsequently, more approachable, before we'll be able to learn much from them. They're still loads of fun to play with, though.