Modules add functionality to your Drupal powered site. You can add a forum to your site by installing the forum module: http://drupal.org/documentation/modules/forum. Mobile platforms are getting more and more powerful and popular. Drupal has reckognized this fact. By installing the Drupal mobi loader http://drupal.org/project/mobi_loader module and the mobile theme http://drupal.org/project/mobile_theme your site will render better on mobile platforms. You can also integrate your own code. We reccomend though that you don't modify the drupal core: http://drupal.org/project/drupal even if it is possible and sometimes tempting.
Source: http://www.packtpub.com/drupal-7-create ... yling/book Page 10.The best-practice approach to customizing your Drupal site involves intercepting and overriding files and styles, that is, creating new code or styles that the system will display in place of the default code or styles.
A note of warning is in place before you download and install modules and themes:
You need to be quite careful about the Drupal version—or indeed modules and themes—you download, because each successive version makes changes and improvements on previous versions, and also sometimes messes up compatibility with other features.
Use the Filter by compatibility block
http://drupal.org/project/Modules
to limit module results to only the version you require.
2. Some popular themes:
http://drupal.org/project/nice_menus
http://drupal.org/project/simplemenu
http://drupal.org/project/megamenu
Find more on the theme page: http://drupal.org/project/themes
3. Some useful modules:
http://api.drupal.org/api/drupal/module ... or.module/ See http://drupal.org/node/627270
http://api.drupal.org/api/drupal/module ... ay.module/
http://api.drupal.org/api/drupal/module ... og.module/ See also http://drupal.org/developing/api/database
Find more on the module page: http://drupal.org/project/modules
4. Integrating your own code.
Develop for Drupal: http://drupal.org/documentation/develop
http://drupal.org/node/360052
4.1. Php
http://drupal.org/coding-standards
http://drupal.org/documentation/modules/php
http://drupal.org/documentation/customi ... p-snippets
http://drupal.org/node/129487
Example:
http://drupal.org/node/244469
4.2. Javascript
http://api.drupal.org/api/drupal/includ ... al_add_js/
http://drupal.org/node/751740
http://drupal.org/project/jquery_plugin
http://drupal.org/node/89005