Logic Bombs
While a logic bomb is not something of pressing concern for one's personal security online, it is still an intriguing concept that falls under the umbrella of malicious software. A logic bomb is a program (or portion of a program) that remains dormant until activated, usually by time or date.
The most common use of a logic bomb is to ensure that software is paid for. When software is not purchased by a certain date (e.g. during a free trial), the software deletes itself. More malicious logic bombs will also delete other data.
Logic bombs tend not to be a major concern for users, as they are not commonly used in attacks, do not proliferate, and cannot be spread to unintended victims in the way that spyware or viruses can. That said, even if they aren't directly threatening to your personal security, they are still an interesting subsection of malware.
Resources
"What is a logic bomb?" Tech FAQ. Accessed April 15, 2007. <http://www.tech-faq.com/logic-bomb.shtml>
Baratz, Adam & McLaughlin, Charles. "Malware: what it is and how to prevent it." Ars Technia. November 11, 2004. <http://arstechnica.com/articles/paedia/malware.ars/>