In C++ a functor is the name of a special kind of operator overloading. Defining a functor for a class allows you to treat an object as if it were a function. This can be confusing, so is rarely seen. An example would be:

class CSquare
    int operator()( int i )
    { return( i * i ); }

Then, using the above class definition, you could write something like the following:

int main()
    CSquare square;

    int s = square( 5 );

One possible use for functors is for multiple subscript arrays (i.e. matrices). Since overloading the bracket operator only allows a single parameter inside the brackets, you could instead define a functor that taks multiple subscripts.