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?

>>3
int add(int a, int b) {
 if(a==1 && b==1) return 2;
 else if(a==-1 && b==-1) return -2;
 else if(a==-1 && b==1) return 0;
 else if(a==2 && b==3) return 5;
}

TDD is shit.