Maybe the least used feature in all of C++. Used mostly by crazed MFC developers and rarely ever practical in real code. Used to avoid repetition in class hierarchies where the lower levels use multiple inheritance and the higher levels inherit from the same base class.
As in : A -> B.
A -> C.
B+C -> D. Class B and C must derive A virtually.
See also: templates as template parameters, explicit instantiation, placement new for other obscure features of the language.