# is the one of a symbol used by a developer of C language
standards for defining of macros... if you are designing a
compiler you can use a different symbol or may not use these
symbols for definition of macros............ it dosen't have
meaning and since # is a rare symbol used . so it's my guess .

the include words includes the header or any file with
extension to the C source code that the run time or linker
time..... in which may inbuilt functions are available like
printf,clrscr(),scanf() etc etc............


thank u

# is a symbol, that represent to the complier,the header
files are includes in the program.....include key word is
using to include the header files in c

# refers to pre-processor directives. Pre-processor word 
means that the header file following #include is already 
processed and their is no chance of it containing any 
errors or flaws.  User can directly include the header file 
in his/her program.

The symbol '#' is called preprocessor.

All the header files are stored in a folder namely INCLUDE. 
When need to us use them we must mention as follows.
#include<headerfile_name.h>

# is the symbol used as the syntax of the header
statement.include is the key word used to link for the
functions in the file(may be in the library or the user
defined) to be used in the program.
 For instance,
               #include<stdio.h>/*used to include the
library file for standard input/output statements-printf()
and scanf()*/

'#' is a preprocessor directive. The compilers processes
lines in a code starting with a '#' before it compiles the
whole code . Example #define TEN 10 the compiler would
replace all instances of TEN by 10 in the code and only then
compile the code . Similarly ,
 
'#include' - include is a key work which ask the compiler to
also resolve function names and variables names using the
file name specified.
Ex '#include<xyz.h>' indicates the compiler that while
searching resolve function names and/or variable names from
xyz.h file also.

The # is the Pre-Processor. Means it is a special kind of 
program that executed before the actual compilation of the 
program. When it executed it performs several tasks like 
defining etc. On such operation is the include. The include 
tell that the current program requires certain inclusion of 
files. Like #include<stdio.h>. Here we are directing the 
pre-processor to include the stdio.h header file.

# is the main part of the 'c'program.it is used to define 
the header file it means that it is used to declare that
header filesfor ex:"#include<stdio.h>".here it is used for 
standarad for declaring printf and scanf() 
functions.here "#" is the main part. 
"#include<conio.h>"
it is used for clrscr() and getch()functions.here 
also "#"is the main header file.actually it is preprocessor.

For example,# is probably known as the pre-processor,because
its called conditional platform code.And also its a header
part section.
The #include use in 2 ways.One is the #include<filename> and
second is the #include"filename".The First one,if the file
is not  found there the compiler checks the source header
directory.If the file is not still found there,the
pre-processor checks the current directory....

#include"filename":
              If the file is not found there the compiler
checks the compile include directory.If the file is still
not  found there the pre-procesor checks the source header
directory. 

Note:
    The compiler first checks the compile include directory
for the specified file.

   And also thanks for giving the chance for scribble here.

# work as a preprocessor when include is used for to connect
the header file to the complier, we want to write
#include"stdio.h" or <stdio.h> because c has no built-in
function to run the program.like wise we include "conio.h"
which is used for link the clrscr()and getch(),but it is not
so important because we can run the program without using
clrscr() and getch() from the header file "conio.h" but we
never run the program without using <stdio.h>

Header file used for to accepped the total programs and
steps in c language. 

                   #include<stdio.h>

#--> preprocessor (define a programing checking concept)

include-->  it interface both the header file and preprocessor
 
<    >-->   Expretion of header file area

"    "-->   charater type using a section of header file.

stdio.h-->  standard input output.header (header file name)