What is the difference between declaring a variable and defining a variable? in C programming language

What is the difference between declaring a variable and defining a variable?

Declaring a variable means describing its type to the compiler but not allocating any space for it. Defining
a variable means declaring it and also allocating space to hold the variable. You can also initialize a variable
at the time it is defined. Here is a declaration of a variable and a structure, and two variable definitions, one
with initialization:

extern int decl1; /* this is a declaration */
struct decl2 {
int member;
}; /* this just declares the type--no variable mentioned */
int def1 = 8; /* this is a definition */
int def2; /* this is a definition */

To put it another way, a declaration says to the compiler, “Somewhere in my program will be a variable with
this name, and this is what type it is.” A definition says, “Right here is this variable with this name and this
type.”

NOTE : One way to remember what each term means is to remember that the Declaration of Independence
didn’t actually make the United States independent (the Revolutionary War did that); it just stated
that it was independent.

A variable can be declared many times, but it must be defined exactly once. For this reason, definitions do
not belong in header files, where they might get #included into more than one place in your program.
 
Cross Reference:

II.17: Can static variables be declared in a header file?