Google uses a combination of algorithms and reporting tools to protect against spam, viruses, and phishing attacks. Using the same advanced computing infrastructure as its search engine, along with a vast community of users, Google touts having one of the best spam blockers in the business.
For more information on Google and spam, click on http://www.google.com/mail/help/fightspam/spamexplained.html.
ref: http://www.hawaii.edu/infotech/spam/spam.html#whatisits
