Two of the key features of the Internet, paradoxically, are that everything is anonymous and nothing is anonymous.
Like some sort of spooky quantum state, both are true at the same time.
How so?
When you read an e-mail or a forum post, it's impossible to know if the purported sender is genuine merely by looking at it. It's a simple matter to add professional looking logos or spoof the "from" like of an e-mail.

It brings to mind one of my favorite cartoons of all time. Two dogs are sitting in front of a computer. One says to the other, "On the Internet, nobody knows you're a dog."
The clever cartoon is by Peter Steiner and appeared in the July 5, 1993 issue of The New Yorker.
While this is all true, prosecutors or plaintiffs armed with subpoenas, shadowy people chasing terrorists and marketers assembling far-reaching databases can indeed know many things about us. While identity information is beyond your grasp and mine, these people know a lot about you and the amount is growing and is never erased.

In light of this some person unknown to me updated the cartoon, adding to the screen "Welcome canine, User 39... Mutt, Mostly black lab. Enjoys pepperoni, fetching, and sniffing other dogs' heinies... Updating profile"
Two key things we can take away from this:
-- Since the Internet is anonymous and we can't tell who sent something just by looking at it, we must use our brains. Pay attention to the little things like context and spelling. Know that any e-mail from you bank is likely bogus. When there's any doubt at all, call the person or the business (and look up the number yourself; don't use the number in the suspect e-mail).
-- Since the Internet is not anonymous and messages last forever, think really hard before you post anything. You could have to answer for it years later.

The cartoons:
http://www.unc.edu/depts/jomc/academics/dri/idog.html