It doesn’t have to be that complex. I will demonstrate how to create a simple plugin in 20 lines, the plugin can add some HTML to page footer and/or post.  This is an “Hello World”, so I won’t explain the plugin framework in detail but I will cover essential concepts.
This tutorial is for (newbie) plugin developers, it won’t explain how to use WP and how to code PHP.

Creating a plugin

Create a directory “simple-add-to-footer” under wp-content/plugins, then create a file “simple-add-to-footer.php”. Add this code to the newly created page.

<?php
/*
Plugin Name: Simple Add To Footer
Plugin URI: http://localhost/wordpress/
Description: Add HTML snippets to posts and/or page footer.
It is a plugin tutorial using latest plugin API of WP2.7+
Version: 0.1
Author: me
Author URI: http://blog.orangecabin.com/
Disclaimer: Use at your own risk. No warranty expressed or implied is provided.
*/
?>

This is called “Standard Plugin Information”, you can read more about it on http://codex.wordpress.org/Writing_a_Plugin. That is all it needs to be a valid plugin. Now this “plugin” can be activated like other plugins.

Adding something interesting

OK it does nothing till now. Please add this after the plugin info.

add_action('wp_footer', 'simple_add_to_footer_wp_footer');
function simple_add_to_footer_wp_footer(){
	echo '<div>Hello world! '. get_option('blogname') .'</div>';
}

If you activated the plugin now, you can see it print something in the page footer.

What is “action”

Here goes some theory : “action” is one type of the extension points of WordPress, that means you extend an action to do something to WordPress.
add_action is “the interface” between wordpress and your plugin. The 1st parameter “wp-footer” is a simple action which is called to produce page footer. The 2nd parameter is name of you function. And you do what you need in that function.
simple_add_to_footer_wp_footer calls get_option(‘blogname’). It is a WordPress defined API to get blog options, like blog name.

Filter – another type of extension point

Now add this to the file

add_filter('the_content', 'simple_add_to_footer_the_content');
function simple_add_to_footer_the_content($content = ''){
	return $content .'<div>I am '. get_the_title(). '</div>' ;
}

Reload your blog page, now you can see it adds some info to the post.
Filter is another type of extension point. Instead of echo something directly, you get a parameter ($content) from WordPress and you can manipulate it, like upper case or something.

Conclusion

That is all. Here is what we have now:

<?php
/*
Plugin Name: Simple Add To Footer
Plugin URI: http://localhost/wordpress/
Description: Add HTML snippets to posts and/or page footer.
It is a plugin tutorial using latest plugin API of WP2.7+
Version: 0.1
Author: me
Author URI: http://blog.orangecabin.com/
Disclaimer: Use at your own risk. No warranty expressed or implied is provided.
*/
add_action('wp_footer', 'simple_add_to_footer_wp_footer');
function simple_add_to_footer_wp_footer(){
	echo '<div>Hello world! '. get_option('blogname') .' </div>';
}
 
add_filter('the_content', 'simple_add_to_footer_the_content');
function simple_add_to_footer_the_content($content = ''){
	return $content .'<div> I am '. get_the_title(). '</div>' ;
}
?>

You can download source code here: simple-add-to-footer_0.1
OK I know it is not really useful. But this plugin can be easily extend to do interesting stuff, like inserting arbitrary HTML code to page footer. To do that, you need a page for the plugin user to type the HTML they want to insert. The next post will explain how to do that easily using latest WordPress plugin API.