.tpl file output using theme function

So, you have prepared the output in one of your module function with all complicated business logic client specified in last meeting.  Your "return ($output);" throws everything on screen. You became glad to see your code worked finally.
And if Table layout is required you pushed all html in $output variable and theming of your own module page become hectic. Better to go with tpl output.



function hook_menu() {
  $items['url'] = array(
    'title' => 'Title',
    'page callback' => 'menu_callback',
    'access callback' => 'user_is_anonymous',
    'type' => MENU_CALLBACK,
  );
  return $items;
}

function menu_callback() {
  $output_var1=your logic;
  $output_var2=your 2nd logic;
  $result = theme('theme_key', $output_var1, $output_var2);
  return($result);
}

function hook_theme() {
  return array(
    'theme_key' => array(
      'arguments' => array('output_var1' => NULL, 'output_var2' => NULL),
      'template' => 'tpl_file_name',
    )
  );
}
Now create your tpl file, and use $output_var1, $output_var2 for output there. and give that tpl to your designer. S/He will love you :-).
For detail understanding look into user module.