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There are two kind of variables

1. Local Variables
2. Global Variables

Local Variables
              These variables are declared with in the block and can not be referenced from outer block.

Global Variables
               These variables are declared in a outer block and can be referenced with in other blocks. Example
DECLARE
VAR_NUM1 number;
VAR_NUM2number;
BEGIN
VAR_NUM1 number := 10;
VAR_NUM2 number :=10;
DECLARE
VAR_ADD number;
BEGIN
VAR_ADD := VAR_NUM1 + VAR_NUM2 ;
END;
END;
/
  Explanation
 First of all we declared two global variables named " VAR_NUM1 and VAR_NUM2 ".Than we declared one more variable named " VAR_ADD " which has a Local scope.At the end we used to times " END " keyword.It means it is a nested Block now we can access the global variable after the last keyword written END but the Variable " VAR_ADD " can not be accessed after first Written keyword " END " because it is a Local Variable.