C Primer Plus, Fourth Edition

File Inclusion: #include

When the preprocessor spots an #include directive, it looks for the following filename and includes the contents of that file within the current file. The #include directive in your source code file is replaced with the text from the included file. It's as though you sat down and typed in the entire contents of the included file at that particular location in your source file. The #include directive comes in two varieties.

#include <stdio.h>	←Filename in angle brackets
#include "mystuff.h"	←Filename in double quotation marks

On a UNIX system, the angle brackets tell the preprocessor to look for the file in one or more standard system directories. The double quotation marks tell it to first look in your current directory ...