Any type of program intended to harm a PC is considered malware. Malware includes:
Viruses
Viruses are programs attach themselves to another program and begin to replicate themselves and spread to other computers. They are spread mostly through the internet and e-mail but can also be passed by infected diskettes and software. These viruses can cause your computer to slow down, corrupt files, and erase information.
Worms
Worms, unlike viruses, are self contained. They do not need other programs to duplicate themselves and often go unnoticed until significant memory space is used.
Trojans
Trojans are hidden coding in programs and data that corrupt files and the PC hard drive. They can not duplicate themselves but they can activate other malware.
Spyware
Spyware is computer software that takes information,such as browser patterns and system information, about a specific computer. It can then redirect you to other web sites or allow pop-up ads to appear. It is not intend to damage your PC it is just an illegal process to gather information.
Adware
Adware is a branch of spyware. It is application that is displayed while another program is running. An example of adware is pop-ups. Adware is also not intended to harm your PC but the extra memory it take to run can slow down your computer's performance.