The amount of people who fall for spam e-mail is enormous, I personally don't know why - because 99.9-percent of the time it's so blindingly obvious, but there are still quite a few who fall to the trick of "you've just inherited $4.5 million from an Uncle you've never heard of, in a country you've never heard of".
Google now want you to know why the messages they mark as spam, and will be, starting as of today, showing a brief explanation at the top of each spam message. All you have to do is look at any message in your spam folder to find out why it was put there, and to learn of any potential harmful content within the message itself.
Google hope this is not only interesting, but helps you learn just how they filter out those bad e-mails. You can leave the spam folder untouched (like most people do), or if you want to get into the educational side of things, the information is there for your help. I don't think I'll bother, I'll let Google sort out my spam, thanks, Google.