Spam is unsolicited e-mail. It is often commercial in nature and may contain viruses or other malware.
Legitimate, non-spam e-mail is sometimes called ham.
The best way to protect yourself from receiving spam is to never give out or publish your e-mail address.
Gustavus offers server-side spam control through a program called SpamAssassin. The spam filtering settings page offers a web-based interface to configure these settings for your e-mail account.
Some e-mail clients like Apple Mail and Mozilla Thunderbird offer built-in spam detection.