有关虚函数通过基类指针或基类引用实现动态多态性，求指教

示例37：阅读下面程序，写出输出结果。
#include <iostream>                                                   // 编译预处理命令
using namespace std;                                                               // 使用命名空间std

class A
{
public:
         virtual void Show1() const { cout << "A" << endl; }         // 输出信息
         void Show2() const { cout << "A" << endl; }                  // 输出信息
};

class B: public A
{
public:
         void Show1() const { cout << "B" << endl; }                  // 输出信息
         void Show2() const { cout << "B" << endl; }                  // 输出信息
};

void Fun1(A obj) { obj.Show1(); }                                             // 定义Fun1()                  
void Fun2(const A &obj) { obj.Show1(); }                           // 定义Fun2()

int main()                                                                                 // 主函数main()
{
         B obj;                                                                        // 定义对象
         A *p = &obj;                                                               // 指针
         p->Show1();                                                               // 输出信息
         p->Show2();                                                               // 输出信息
         Fun1(obj);                                                                        // 调用Fun1()
         Fun2(obj);                                                                        // 调用Fun2()
         
         return 0;                                                        // 返回值0, 返回操作系统
}
请问这程序是怎么理解的，我感觉很混乱，不好记住，求指教