form validation rule question

[eluser]vps4[/eluser]
what different with

alpha_numeric
numeric
is_numeric
integer
is_natural
is_natural_no_zero

it seems they are same thing.

$lang['alpha_numeric'] = "The %s field may only contain alpha-numeric characters.";
$lang['numeric'] = "The %s field must contain only numbers.";
$lang['is_numeric'] = "The %s field must contain only numeric characters.";
$lang['integer'] = "The %s field must contain an integer.";
$lang['is_natural'] = "The %s field must contain only positive numbers.";
$lang['is_natural_no_zero'] = "The %s field must contain a number greater than zero.";