What is Spam?
Spam is the unwanted junk in your email inbox that was sent to you without permissions by someone you don't know. This is a broad term that includes offers for Viagra, Cialis, Human Growthe Hormone, "adult" stuff related to genitalia, offers to invest in an offshore scam involving a large sum of money, home loans from sketchy banks, announcements of stock offers from firms you don't do business with, phishing emails requesting you to login to your bank and much more.
The major difference between legitimate commercial email and spam, is that you didn't ask (opt-in) for the Spam and it's usually some sort of scam or offer for products and services.
Other terms of note:
UBE - Unsolicited Bulk Email, a common name for Spam emphasizing that it's sent en masse and without being asked for.
UCE - Unsolicited Commercial Email, a common name for Spam emphasizing its commercial nature and without being asked for.
Phishing Emails are emails that looks much like a legitimate email from your bank, Paypal, eBay, or other password-protected site that you may or may not have an account with. If you receive an email to login to an account to update information, etc, be sure to not click on any links in that email. First, check with the alleged sender's website to and read up on their phishing email warnings.