[求助]exists的用法

请高手能详细讲一下exists的用法,我对exists的用法还很模糊,谢谢!
USE pubs
GO
SELECT DISTINCT pub_name
FROM publishers
WHERE EXISTS
(SELECT *
FROM titles
WHERE pub_id = publishers.pub_id
AND type = 'business')
上面例子的具体含义.如果我把它改成
SELECT DISTINCT pub_name
FROM publishers
WHERE EXISTS
(SELECT *
FROM titles)
又有何含义呢?