Computer Viruses, spyware, malbots and other programs which are designed to create minor or major disruptions to a computer system. Malicious Software can be obtained from a vareity of sources; although it is most commonly associated with being obtained from using the Internet.  

Other known sources of obtaining malicious software include email attached, running programs on CD/Flash drives which haven't been cleaned or verfied or even dangerous executables that can occur in macros from programs like Microsoft Word or Excel.