Difference between Declaration and Definition

Declaration:-

• It only introduces the name of an identifier along with its type to the compiler before it is used.
• During declaration, no memory space is allocated to an identifier.

Definition:- 

• Definition of an identifier means the declaration of an identifier also reservation of space for it in the memory.
• The amount of memory space reserved for an identifier depends upon the data type of an identifier.
• Identifiers of different data types take different amounts of memory space.
• The memory space required by an identifier also depends upon the compiler and working environment used.