Thread: Declarations and definitions in C

Guys what is the difference between declarations and definitions of varibles,functions, structures?
I understand

Code:

int c;
int d;

are examples of declarations and int c=4; is an initilisation but how does variable definition differ?

Same with structures

Code:

struct autopart
{
char id;
float price;
}

//is the structure declaration.What is the definition of a structure.

for functions

Code:

int sum2digits(int a,int b);  //the prototype

lets say...

Code:

int sum2digits(int a, int b)  //what is this line called?
{
return a+b;
}

what are the definitions and declarations.