What is the difference between #include <filename> and #include “filename”?

The difference between the two forms is in the location where the preprocessor searches for the file to be included.

#include <filename>

The preprocessor searches in an implementation-dependent manner, it searches directories pre-designated by the compiler. This method is usually used to include standard library header files.

#include "filename"

The preprocessor searches in the same directory as the file containing the directive. If this fails, then it starts behaving like the #include <filename> form. This method is usually used to include your own header files.