What is spam?

Wikipedia defines it as: "Spamming is the abuse of electronic messaging systems to send unsolicited bulk messages, which are generally undesired. "
So in short terms, spam can be defined as all the emails you receive that come from people you don't know. Most common examples are emails for false diplomas, selling Viagra or emails telling you to buy this or that stock option.
Just the same as paper publicities that you receive every day in your mailbox, spam fills the need for some products to be seen. The trouble with spam, is that represents more than 70% of all emails sent in the world. Even thought it doesn't waste any paper, the proportion it has taken makes it annoying.

Search engines (Google, Yahoo...) have some programs named spiders that go through WebPages by following hyperlinks. That way they can map how the Internet is shaped.
Spammers have the same programs, except that their spiders collect all email addresses they find on their way. To do so is pretty easy since the @ character is only used in email addresses.

It is not that hard to avoid spam, but you have to be careful.

The first thing you should do, do not make your email address public (visible). So do not put it on a website or even in a post in a forum. It is why our forum uses private messages to let members get in touch. Same thing with EBay or Craiglist, the seller never shows his email address on a webpage. If you do it, it is on your own risk !

Also, avoid giving your email address on any websites requiring email address to receive special offers or Newsletters… there is a good chance that you will receive spam related to that. Those websites can share or sell your email address. If you feel that the emails you receive from them are not secured (all recipients email addresses are visible in the email), you should unsubscribe.