求help!一早上在纠结main 加_t的问题，帮忙解释一下！！

#include "stdafx.h"
#include <iostream>
using namespace std;
          /*
int main(int argc, char* argv[])
{
     char* a[] = {"ABC"};
     cout << "argv[1] = " << argv[1] << endl;
     cout << "a[0] = " << a[0] << endl;
     system("PAUSE");
     return 0;
}
         */
/*-------------------------
结果：
argv[1] = readme.txt
a[0] = ABC
请按任意键继续. . .
-----------------------*/
//         /*
int _tmain(int argc, char* argv[]) //这里mian前面加了"_t"
{
     char* a[] = {"ABC"};
   //const char* b[] = {_T("ABC")}; //这个出错转型失败
     const wchar_t* b[] = {_T("ABC")};
     cout << "argv[1] = " << argv[0] << endl;
     cout << "a[0] = " << a[0] << endl;
     cout << "b[0] = " << b[0] << endl;
     system("PAUSE");
}
//         */
/*-------------------------
结果：
r
a[0] = ABC
b[0] = 004179E8
请按任意键继续. . .
-----------------------*/