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From C++ in a Nutshell:

A base class can be declared with the virtual specifier, which is important when a class has multiple base classes. Ordinarily, each time a class is named in an inheritance graph, the most-derived class is organized so that an object has a distinct subobject for each occurrence of each base class. Virtual base classes, however, are combined into a single subobject. That is, each nonvirtual base class results in a distinct subobject, and each virtual base class gets a single subobject, no matter how many times that base class appears in the inheritance graph.

What does this mean? Let's start with an example:

In C++, the virtual keyword, when applied to class methods, aids in polymorphism. If a method is declared to be virtual, the most derived version of the method is executed when a call is made.

If a method is non-virtual, the specific version that is called depends on how the method is called. If the method is called via a pointer to the base class, the base class method is called, if by the derived class, then the derived method is called.

This definition is weak; an example is better, as usual:

#include <iostream>
#include <string>
 
struct Base