在导出的DDL中key为何意？

ayalastrike

CREATE TABLE `customer_news_letter` (
  `customer_news_letter_id` int(11) NOT NULL,
  `customer_id` int(11) NOT NULL,
  `news_letter_schedule_id` int(11) NOT NULL,
  `subscribe_date` datetime NOT NULL,
  `unsubscribe_date` datetime default NULL,
  `send_format_type_id` int(11) default NULL,
  PRIMARY KEY  (`customer_news_letter_id`),
  KEY `FK_customer_news_letter_type_id` (`send_format_type_id`),
  KEY `FK_customer_news_letter_schedule_id` (`news_letter_schedule_id`),
  KEY `customer_id` (`customer_id`),
  KEY `subscribe_date_index` (`subscribe_date`),
  KEY `unsubscribe_date_index` (`unsubscribe_date`),
  CONSTRAINT `FK_customer_news_letter_1_1` FOREIGN KEY (`news_letter_schedule_id`) REFERENCES `news_letter_schedule` (`news_letter_schedule_id`),
  CONSTRAINT `FK_customer_news_letter_type_id` FOREIGN KEY (`send_format_type_id`) REFERENCES `types` (`type_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

这个里面我知道PK是customer_news_letter_id
FK是news_letter_schedule_id和send_format_type_id
但是里面的这些KEY是什么呢？
  KEY `FK_customer_news_letter_type_id` (`send_format_type_id`),
  KEY `FK_customer_news_letter_schedule_id` (`news_letter_schedule_id`),
  KEY `customer_id` (`customer_id`),
  KEY `subscribe_date_index` (`subscribe_date`),
  KEY `unsubscribe_date_index` (`unsubscribe_date`),

realzyy

key就是索引