Wordpress functionality can be customized through the use of Wordpress Plugins. If you have a functionality that you want in your wordpress site that can not be found in an existing plugin, you can write your own plugins with a bit of PHP knowledge.

Here are some tutorials to get you started on creating your own Wordpress plugins.

• Wordpress Codex on Writing Plugin: Best place to start off with is the Wordpress Codex documentation as to how Wordpress plugin works and how to create one.
• Video for First Wordpress Plugin: Mark Jaquith shows us in this video as to how to make a bare minimal wordpress plugin. The plug-in uses the "add_filter" hooks to "the_content" and "the_title" to replace words of "foo" with "bar".
• Wordpress Plugin from Scratch -- This tutorial not only shows you how to create a Plugin from the beginning, it also shows more sophisticated use of accessing the database. As of this writing, there appears to be a typo in the code. The call to "get_results" should be "$results=$wpdb->get_results".
• Adding Administration Menus: When your plugin gets more complicated, you may have the need to add administrator menus.