<p>I want to know what a "<em>virtual base class</em>" is and what it means.</p>

<p>Let me show an example:</p>

<pre><code>class Foo
{
public:
    void DoSomething() { /* ... */ }
};

class Bar : public virtual Foo
{
public:
    void DoSpecific() { /* ... */ }
};
</code></pre>

It means a call to a virtual function will be forwarded to the "right" class.
C++
FAQ Lite
FTW.
In short, it is often used in multiple-inheritance scenarios, where a "diamond" hierarchy is formed.  Virtual inheritance will then break the ambiguity created in the bottom class, when you call function in that class and the function needs to be resolved to either class D1 or D2 above that bottom class.  See the
FAQ item
for a diagram and details.
It is also used in
sister delegation
, a powerful feature (though not for the faint of heart).  See
this
FAQ.
Also see Item 40 in Effective C++ 3rd edition (43 in 2nd edition).