what is the difference between definition and declaration in c

A declaration introduces an identifier
and describes its type, be it a type, object, or function. A declaration is what
the compiler needs to accept references to that identifier. These are declarations:

extern int bar;
extern int g(int, int);
double f(int, double); // extern can be omitted for function declarations
class foo; // no extern allowed for class declarations

A definition actually instantiates/implements
this identifier. It's what the linker
needs in order to link references to those entities. These are definitions corresponding to the above declarations:      [a definition allocate space for the identifier   // myself]

int bar;    [someone said it is not only a definition but also a declaration]
int g(int lhs, int rhs) {return lhs*rhs;}
double f(int i, double d) {return i+d;}
class foo {};

A definition can be used in the place of a declaration.

An identifier can be declared as often as you want. Thus, the following is legal in C and C++:

double f(int, double);
double f(int, double);
extern double f(int, double); // the same as the two above
extern double f(int, double);

However, it must be defined exactly once. If you forget to define something that's been declared and referenced somewhere, then the linker doesn't
know what to link references to and complains about a missing symbols. If you define something more than once, then the linker doesn't know which of
the definitions to link references to and complains about duplicated symbols.

[from website] http://stackoverflow.com/questions/1410563/what-is-the-difference-between-a-definition-and-a-declaration