An expanded example of using hooks and classes in WordPress.

class.php

<?php
/**
 * Our main class for our theme.
 * 
 * This will act as our controller, and activate other hooks/functions for wordpress.
 */
 
class Theme {
  
  // Called by our bootstrap_theme function
  public static function init() {
    
    // Our theme will modify wp queries
    self::modify_wp_queries();
    
    // Our theme will modify $post objects
    self::modify_wp_post_object();
    
  }
  
  /**
   * Modify wordpress queries
   */
   public static function modify_wp_queries() {
    require_once locate_template('/lib/theme/queries.php');
    ThemeQueries::init();
   }
    
  /**
   * Add Custom fields to posts
   */
   public static function modify_wp_post_object() {
    require_once locate_template('/lib/theme/posts.php');
    ThemePosts::init();
   }
}

functions.php

<?php
/**
 * Our functions file. We're hooking into the wp after_setup_theme 
 * hook to bootstrap our theme, instead of just firing everything 
 * all willy-nilly.
 */
 
// Bootstrap our theme, this is called by the after_setup_theme hook
function bootstrap_theme {
  
  // Include our class file, which we use to control our theme
  require_once locate_template('/lib/theme/class.php');
  Theme::init();
  
}
 
add_action( 'after_setup_theme', 'bootstrap_theme' );

posts.php

<?php
/**
 * Posts.php
 * 
 * This class is called by class.php
 * 
 * It will hook into the_post() to make post modifications.
 */
class ThemePosts {
  
  public static function init() {
    
    // __CLASS__ allows us to reference this class, in the callback
    add_action('the_post', array(__CLASS__, 'add_custom_fields'));
    
  }
  
  /**
   * Add custom post meta to posts
   * 
   * This function is called by the the_post() hook.
   */
  public static function add_custom_fields( $post ) {
    $post->fields = get_post_meta($post->ID);
  }
  
}

queries.php

<?php
/**
 * Queries.php
 * 
 * This class is called by class.php
 * 
 * It will hook into pre_get_posts to make post modifications.
 */
class ThemeQueries {
  
  public static function init() {
    
    // __CLASS__ allows us to reference this class, in the callback
    add_action('pre_get_posts', array(__CLASS__, 'modify_home_query'));
    
  }
  
  /**
   * Modify Homepage Query to custom post type
   * 
   * This function is called by the pre_get_posts hook.
   */
  public static function modify_home_query( $wp_query ) {
    if ($wp_query->is_main_query() && is_home() ) {
      $wp_query->set('post_type', 'custom');  
    }
  }
  
}