What is a man-in-the-middle attack?

A “man-in-the-middle” attack makes you think you are talking directly to a trusted Web site, say your bank.  But in fact there is someone in the middle reading and then relaying the messages to and from the site.  The man-in-the-middle can steal login and password information, or hijack your session, keeping it open after you think it is closed.  

Evil twin, phishing and Internet address redirects are all attacks that can lead to a man-in-the-middle.  (See also, What is spyware and malware?)