There's already a number of answers for this question but I have tried to provide a very simplistic approach. Hopefully identifying to devs the array structure expected by Drupal when returning your block content.

To do this I have broken the question down into separate code examples as such,

/**
 * Implements hook_theme().
 */
function examplemodule_theme() {
  return array(
    'examplemodule_output' => array(
      'variables' => array(
        'title' => NULL,
        'content' => NULL,
        'popular_content' => NULL,
       ),
      'template' => 'templates/examplemodule-sweet--block',
    ),
  );
}

Please see a full explanation here drupal 7 creating theming custom blocks

There's already a number of answers for this question but I have tried to provide a very simplistic approach. Hopefully identifying to devs the array structure expected by Drupal when returning your block content.

To do this I have broken the question down into separate code examples as such,

/**
 * Implements hook_theme().
 */
function examplemodule_theme() {
  return array(
    'examplemodule_output' => array(
      'variables' => array(
        'title' => NULL,
        'content' => NULL,
        'popular_content' => NULL,
       ),
      'template' => 'templates/examplemodule-sweet--block',
    ),
  );
}

Please see a full explanation here drupal 7 creating theming custom blocks