Drupal Clean URL is a good feature which rewrites the URL and allow you to write search engine friendly URLs. This should have mod_rewrite enable. If you are running your site on shared hosting where you dont have control on the httpd.conf file you can modify you .htaccess file to enable this feature. See default .htaccess file from durpal for more detail. The feature can be enable from configuration menu in drupal 7 using link "Clean URLs". You see a button to test if your server allow it or not. If your test success you see a checkbox. Click on the checkbox and save configuration.

Sometime even if your server is capable of doing it after making the required changes as describe at https://drupal.org/node/15365 you dont see the checkbox.
The reson for that could be the the way (URL) you are accessing the config page ie http://yoursite.com/?q=admin/config/search/clean-urls change URL to 'http://yoursite.com/admin/config/search/clean-urls' you will see the checkbox click on it and save. 

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