Why does the linker give me an error in the virtual case, but not in the non-virtual case?

When I have something like this:

class A
{
  virtual void rat();
};

class B : public A
{
  virtual void rat() { ; } //implemented!
};

int main(int argc, char **argv)
{
  A *a = new B;
  delete a;
}

I get linker errors:

unless I make the base rat pure virtual.

However, when I have this:

class A
{
  public:
  void rat();
};

int main(int argc, char **argv)
{
  A a;
}

This compiles fine and does not give me a link to an undefined link unless I explicitly try to call the rat function in my main ( a.rat();). What is the rule for unrealized virtual functions of a base class, which, however, are implemented in a derived class, as in the first code fragment with an error?