Unit Tests

Can somebody explain unit testing and test driven development? I've tried to learn it through blogs and code samples and such, but I just can't grasp it. It seems like a really backwards way of writing code. I've never really had a problem learning all kinds of programming concepts and methodologies in the past but this stumps me. Is it supposed to be a really difficult thing to learn or am I just retarded?

using asserts gives both the advanages of debugging and testing. You just verify parts of your code are giving the results you expect with asserts, it eliminates the need to make driver code to run your program or having to step through it with a debugger