Hello Sridhar,
This query should have been posted in Crystal reports space. However, to answer your question.
Local variable is something whoes scope is limited to a single formula and you cannot use that variable outside that formula.
Global Variable's scope is throughout the report means you can access the variable's value anywhere in the report except subreport.
Shared Variable is something that overcomes the limitations of Global variable and used to share across the main and sub report as the name suggests.
Regards
Niraj
