Junk email is the electronic form of "standard" junk mail with one
important difference: the recipient, instead of the junkmailer, pays
for the message.
Technically, this form of net-abuse is known as UCE (short for
Unsolicited Commercial Email).
Usually, your email address is gathered from Usenet news spools (try it:
search for your own name on DejaNews.
You'll be surprised ;)
What is the difference between junk email and spam?
Well, junk email is sent, via email, to individual users (usually, many
thousands at a time). Spam is posted to Usenet news servers all around
the world (take a look at spamming for more information).