How to make new widget in Wordrpress? Explain with examples.

class My_Widget extends WP_Widget {
         public function __construct() {
               // widget actual processes
            parent::WP_Widget(false,'Tags by Category','Simple widget to extract tags by Category');
        }

        function form( $instance ) {
        $instance = wp_parse_args( (array) $instance, array( 'title' => '') );
        $title = $instance['title'];
?>
        <p><label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?> <input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo esc_attr($title); ?>" /></label></p>
<?php
    }

    function update( $new_instance, $old_instance ) {
        $instance = $old_instance;
        $new_instance = wp_parse_args((array) $new_instance, array( 'title' => ''));
        $instance['title'] = strip_tags($new_instance['title']);
        return $instance;
    }

        public function widget( $args, $instance ) {
               // outputs the content of the widget
            $title = apply_filters( 'widget_title', empty( $instance['title'] ) ? '' : $instance['title'], $instance, $this->id_base );

        echo $before_widget;
        if ( $title )
            echo "<h3 class='widget-title'>".$before_title . $title . $after_title."</h3>";
           
             query_posts('category_name=written-articles');
 if (have_posts()) : while (have_posts()) : the_post();
        $posttags = get_the_tags();
  if ($posttags) {
   foreach($posttags as $tag) {
    $all_tags_arr[] = $tag -> name; //USING JUST $tag MAKING $all_tags_arr A MULTI-DIMENSIONAL ARRAY, WHICH DOES WORK WITH array_unique
    $all_link[] = get_tag_link($tag->term_id);
   }
  }
 endwhile; endif;

 $tags_arr = array_unique($all_tags_arr); //REMOVES DUPLICATES
 $tags_link = array_unique($all_link);
 //echo '<pre>'.print_r($tags_arr, true).'</pre>'; //OUTPUT FINAL TAGS FROM CATEGORY
 //echo '<pre>'.print_r($tags_link, true).'</pre>';
echo '<div class="tagcloud">';
    for($i = 0; $i<count($tags_arr);$i++)
    {
        echo '<a href="'.$tags_link[$i].'">'.$tags_arr[$i].'</a>'." "; 
    }

        echo "</div>\n";
        }

}
register_widget( 'My_Widget' );