Первое — это include собственного заголовка, то есть искать его в рабочем каталоге, в дополнение к стандартным каталогам компилятора.
#include <file>
This variant is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the -I option (see Invocation).
#include "file"
This variant is used for header files of your own program. It searches for a file named file first in the directory containing the current file, then in the quote directories and then the same directories used for <file>. You can prepend directories to the list of quote directories with the -iquote option.
