From Spamwiki

[edit] Description

A type of attachment spam, greeting card spam has victims click on what they believe to be an electronic greeting card from someone they know. The URL is actually a means of transmission for computer contaminants.

In July 2007, Symantic reports that over 250 million customers were targeted with this type of spam.

[edit] Sources

http://www.symantec.com/enterprise/security_response/weblog/2007/08/august_state_of_spam_report.html