Use the preprocessor to:

1) define constants:

     #define DIM 25

2) include code (typically header files):

     #include <stdio.h>

3) to conditionally include or exclude (comment out) code such as when
   debugging or to prevent the same typedef from appearing twice:

     #ifndef MYHEADER
     #define MYHEADER
        typedef struct xxx MYTYPE
     #endif 

4) to define macros, but always bracket the expression and all parameters
   to prevent errors:

     #define SQR(x) ( (x) * (x) )

   No space is allowed between SQR and ( in the #define, but elsewhere and 
   in use spaces are allowed.

WARNINGS:

Macro expansion is not part of the syntax of the language - the preprocessor
does not "understand" C, and so some strange looking things can be 
done with it.  Avoid the temptation.

Macros are not functions.  Avoid using a macro on an argument that has 
side effects if the definition uses the argument more than once.
SQR(a++) is really  ((a++)*(a++)) which double increments "a".
getc() is of this type - use fgetc() instead when side effect are possible. 

Never use the preprocessor to define macros for anything other than
expressions, as errors can occur if you define statements because in 
statements a user may not know if nothing, ";" or "}" is the appropriate 
terminator for your macro.
This can cause problems especially in connection with the "else" part of "if" 
statements.  If conditionals are required, use the expression version ":?"
not an if-else statement.

DEBUGGING:

Compile with the -E flag to see the output of the preprocessor.

ARCANE:

__FILE__  is file name
__LINE__  is line number  (there are other __ predefined macros)
#name     is stringification (turns name into a string)
##        is concatenation (turns adjacent strings into a single token)
       
For example,  #define SOURCENAME(file)  #file ## ".c"
makes  SOURCENAME(hello)  into string "hello.c"        

Another example is assert(exp) which is a macro that evaluates expression
"exp" and if false aborts the program with a message generated by function 
__assert().   The source location of the stament is printed.
Here are two possible definitions for assert:

#define assert(exp) ((expr) ? (void)0 : __assert(#expr,__FILE__,__LINE__))
#define assert(exp) ((void)((expr)  ||  __assert(#expr,__FILE__,__LINE__))