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I am new at joomla code understanding and trying to understand from scratch.
I think this may also help other new comer at joomla 1.5
Small explanation from any joomla development expert will works a lot.
First from first,

Any body please explain in short, 

define( '_JEXEC', 1 );


What is the importence of  _JEXEC

Thanks a lot

The _JEXEC variable is used for source code protection.

The variable is set when the site is initialized.
In several important files there is a check for this variable to be set to 1. If it is not set, the application is not initialized. 

For example if you approach the template file (yoursite.com/templates/template/index.php) directly in your browser, it would give a notice that the file cannot be accessed directly.

This way files cannot be accessed directly but only from within the application.

_JEXEC_ is a constant initialized in order prevent unauthorized access (inclusions) to package' files. As MvBrakel said you find it everywhere in Joomla!

Please note Joomla! also feature the JPATH_BASE constant, "to ensure this file is within the rest of the framework". Two different approaches, same result.

Greetings.

Thanks to all, Now I moving towards next step.