$var $variable account?

Said that

<?php
function my_module_theme()
{
  return array
  (
    'me_or_them' => array
    (
      'variables' => array('account' => NULL),
    ),
  );
} 
?>

is hook_theme function for registering theme function to drupal. and then theme_me_or_them is registered.

Then could someone please tell me if $variables in

<?php
function theme_me_or_them($variables)
{
  global $user;

  $account = $variables['account'];
  return ($user->uid == $account->uid) ? t('Me') : t('Them');
} 
?>

is taken from hook_theme or not?

as to
$account = $variables['account'];
in

<?php
function theme_me_or_them($variables)
{
  global $user;

  $account = $variables['account'];
  return ($user->uid == $account->uid) ? t('Me') : t('Them');
}
?>

could someone tell me where '$varibles' comes from and where 'account' come from ?

from time to time, we see hook and HOOK, could someone tell us the difference or meaning?

In system, we see $var and $variables. Could some one tell us the difference or inside meanings?

<?php
return array
(
  'my page' => array
  (
    '#theme' => 'me_or_them',
    '#account' => $account,
    '#prefix' => '<p>' . t('This page was created by '),
    '#suffix' => '</p>'
  )
);
?>

Could someone tell me the usage of my page as key to the array ?

Thanks