Other 

"Malware"

Malware is considered any program that has negative intentions toward a computer.  Malware includes viruses, trojans, and worms, along with some others we have not yet talked about. Malware can cause more damage than viruses.

Droppers are used to avoid anti-virus software.  They are programmed to transport and install viruses into computers.  Triggers are often used to tell the dropper when to insert the virus. Some anti-virus programs can detect droppers.

Logic bombs are also malware.  They are programs that stay dormant in a computer until set off by a trigger.  They are put into a category of their own because they do not replicate, and they do not look like anything useful.

Other "stuff"

Bugs are also something worth mentioning even though they are not considered malware.  Bugs are not malware because they are not intentionally harmful.  Bugs occur when a programmer makes a mistake and the program does not function properly. There are bugs in most complex programs.