A virus is a self-replicating piece of software which usually sends itself to other computers via email or the Internet. It does not require human intervention. Its purpose is to either replicate, cause computer damage, or both. It typically comes from infected emails or documents and can either do its damage right away, or be like a ticking time bomb waiting for the special day to activate.
Examples of viruses:
Boot viruses such as Michelangelo and Disk Killer load when the computer reads the disk. This type of virus is extremely difficult to get rid of.
Program viruses attach themselves to the executable programs on the computer and replicate themselves to all executables on the hard drive. Again, these are very difficult to remove. Examples include the Sunday Virus and Cascade Virus.
Stealth viruses manipulate file sizes to avoid detection. Examples include the Whale virus and the Frodo virus.
Polymorphic viruses change when they replicate so they don't look the same to antivirus software or humans attempting to find them. Examples include the Stimulate virus and Virus 101.
Macro Viruses infect Microsoft Office documents (and others) and infects the normal.dot file (the template that opens with Word when you don't open a file). These viruses infect every document that is opened in the program, and replicates itself to other computers when infected files are shared. Examples include the DMV and Nuclear viruses.
Viruses also got really good at doing something else: disabling anti-virus software. Not only could this particular virus do its dirty deeds after this event, but other malware could also infect the computer without fear of being caught. As a matter of fact, on many routine service calls I would observe that the little anti-virus software icon near the clock disappeared, and the computer user never even noticed the difference (at least until I pointed it out!).
Article Source: http://EzineArticles.com/4410265