C - #define

Completely beginner question, so please keep answers as simple as possible:
In C programming, what is the purpose of using the #define directive (eg #define PI 3.14), when you can just declare a variable and initialize it to the same value?

>>22
I think that that ``in LISP'' guy doesn't actually like Lisp to use it. I never had any problem with S-Expressions after I got my copy of Emacs and Paredit, however without them, I can see how someone could find it painful - it's just a matter of knowing the right tools to use, and being informed of their existence early enough when learning the language.