A WordPress Plugin is just a PHP program with a set of one or more functions.

The plugin file must have a header, without this header, the plugin will never be activated. Here is the header format:

<?php
/*
Plugin Name: Name Of The Plugin
Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates
Description: A brief description of the Plugin.
Version: The Plugin's Version Number, e.g.: 1.0
Author: Name Of The Plugin Author
Author URI: http://URI_Of_The_Plugin_Author
*/
?>


And then write the functions below that header.

More information for Wordpress programming please see Programming in WordPress Way