user visits module

HI all i cant understand this code in the .INSTALL file

function user_visits_install() {
  switch ($GLOBALS['db_type']) {
    case 'mysql':
    case 'mysqli':
      db_query("CREATE TABLE {user_visits} (
                  uid int(10) unsigned NOT NULL,
                  vuid int(10) unsigned NOT NULL,
                  visit int(11) NOT NULL,
                  referer VARCHAR(255) NOT NULL default '',
                  KEY uid (`uid`)
                ) TYPE=MyISAM /*!40100 DEFAULT CHARACTER SET utf8 */;"
      );
      break;
    case 'pgsql':
      db_query("CREATE TABLE {user_visits} (
                  uid int int_unsigned NOT NULL,
                  vuid int int_unsigned NOT NULL,
                  visit int NOT NULL,
                  referer VARCHAR(255) NOT NULL default ''
                );"
      );
      db_query("CREATE INDEX {user_visits}_uid_idx ON {user_visits} (uid);");
      break;
  }
}

cant understand y is that $GLOBALS used for ?