Difference between declaration and definition

Declaration
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      Declaration tells the type of the variable only. That is , it tells whether it is an int or a float or a char. But, the space needed for the variable is not allocated during declaration.
int a;
After this statement, the compiler knows that there is going to be a variable a of type 'int'. But no space is allocated.


Definitionl
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        Space is allocated for the variable and some initial value is assigned.
int a=50;
Variable a is initialised and assigned a value. Only during this definition, space for the variable is allocated.

Note
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Redefinition is an error but redeclaration is not an error.