What is the difference between declaration and definition?

A declaration introduces a name and its type (promising it exists somewhere); a definition also provides the implementation/storage. You can declare something many times but define it only once (per the ODR). Header files typically contain declarations; source files contain definitions. Using an undeclared name is a compile error; a missing definition is a link error.

extern int x;      // declaration
int x = 5;         // definition
void f();          // declaration
void f(){}         // definition