Junk e-mail is called Spam. Spam is usually defined as "mass produced unsolicited e-mail" or "mass produced unsolicited commercial e-mail." In other words, it is junk mail.
Most spam messages contain scams, pornography, hoaxes, viruses, or similar things. It is strongly recommended that you do NOT open these. Also, most anti-spam experts suggest that you should NEVER reply to them, because it verifies that your address is valid.
Some Internet Providers now have anti-spam programs built into their service. They are not perfect, but they work fairly well. You can also get an anti-spam program for your computer, but unfortunately I don't have any experience with them, so I can't recommend any. You can also build your own anti-spam filters in most e-mail programs, but this can be quite complex for beginners, and usually they don't stop that much spam.
Spam (Junk E-mail)