Refer to the get_users()
Codex entry.
$users = get_users();
You can pass the function an array of arguments, including:
$args = array(
// Order by username (display name)
// Note: 'login' is default
'orderby' => 'display_name',
// Order (note: 'ASC' is default)
'order' => 'ASC'
);
$users = get_users( $args );