When you will see the source of Site using Joomla 2.5 you will see the following

<meta name=”generator” content=”Joomla! – Open Source Content Management” />

Why to remove Tag

Its good to show to the user who see your source that you are using Joomla. But if you are concerned with the security of your site,you should remove Joomla Generator Tag. Hackers can know the Joomla version you are using. If security hole is found in that version, then hackers can hack your site.

Removing Tag

To remove Generator Tag from Joomla 2.5 put the following code in the current template file “index.php” between <?php  ?> at the top of the file.

$this->setGenerator(null);

http://www.tutsforu.com/how-to-remove-joomla-generator-tag.html