#ifndef

The #ifndef preprocessor directive checks if macro is not defined by #define. If yes, it executes the code otherwise #else code is executed, if present.
Syntax:
#ifndef MACRO //code #endif
Syntax with #else:
#ifndef MACRO//successful code #else//else code #endif

#ifndef example

see a simple example to use #ifndef preprocessor directive. 
#include <stdio.h>    
#define INPUT  
void main() {  
int b=0;  
#ifndef INPUT  
b=4;  
#else  
printf("Enter b:");  
scanf("%d", &b);  
#endif         
printf("Value of b: %d\n", b);  
getch();  
}  
Output:
Enter b:10
Value of b: 10
But, if you don't define INPUT, it will execute the code of #ifndef. 
#include <stdio.h>    
void main() {  
int b=0;  
#ifndef INPUT  
b=4;  
#else  
printf("Enter b:");  
scanf("%d", &b);  
#endif         
printf("Value of b: %d\n", b);   
 }  
Output:
Value of b: 4

#if

The #if preprocessor directive evaluates the expression or condition. If condition is true, it executes the code otherwise #elseif or #else or #endif code is executed.
Syntax:

#if expression//code #endif
Syntax with #else:
#if expression//if code #else//else code #endif
Syntax with #elif and #else:
#if expression//if code #elif expression//elif code #else//else code #endif

#if example

see a simple example to use #if preprocessor directive. 
#include <stdio.h>    
#define NUMBER 10  
void main() {  
#if (NUMBER==10)  
printf("Value of Number is: %d",NUMBER);  
#endif 
   }  
Output:
Value of Number is: 10
see another example to understand the #if directive clearly. 
#include <stdio.h>    
#define NUMBER 5  
void main() {  
#if (NUMBER==1)  
printf("2 Value of Number is: %d",NUMBER);  
#endif  
#if (NUMBER==5)  
printf("3 Value of Number is: %d",NUMBER);  
#endif    
  }  
Output:
3 Value of Number is: 5