使用尖括号表示在包含文件目录中去查找(包含目录是由用户在设置环境时设置的)，而不在源文件目录去查找；
#include""
使用双引号则表示首先在当前的源文件目录中查找，若未找到才到包含目录中去查找；
MSDN的相关说明
#include""
This form instructs the preprocessor to look for include files in the same directory of the file that contains the #include statement, and then in the directories of any files that include (#include) that file. The preprocessor then searches along the path specified by the /I compiler option, then along paths specified by the INCLUDE environment variable.
#include<>
This form instructs the preprocessor to search for include files first along the path specified by the /I compiler option, then, when compiling from the command line, along the path specified by the INCLUDE environment variable.
扩展知识
1.
Q：C++中的预处理#include和#include对程序的运行有什么不同？
A：没有区别
2.
Q：#include后加using namespace std; 和直接用#include在运行上有什么区别？
A：#include是早些时候的形式
最新的标准定义的是
#include
using namespace std;
比如#include变成
#include
using namespace std;
#include变成
#include
using namespace std;