this folder contains header files of the engine. 
header files mean : any header file that will be included by final developers .
in other way , any header file will be distributed with binary version of the engine .


example 1:
 "Random Class" needs two files :
Random.h : in Include folder  , contains  the declarations .
Random.cpp : in source folder ,contains the implementation.

example 2 :
"Graphics Interface" needs three files :
Graphics.h         : in Include folder , contains  the declarations of Graphics Interface .
OpenGLGraphics.h   : in source folder , contains  the declarations of OpenGLGraphics Class .
OpenGLGraphics.cpp : in source folder , contains  the implementation of OpenGLGraphics Class .

summery :
any file that will not be included by the final developer  , put it in the source folder,
otherwise put it in the include folder.