About Viruses
The old saying, "know your enemy", is quite valid. In order to protect yourself, you have to know what you are dealing with, and what you are up against.

"Virus" is a general catch all term that not only refers to true viruses, but also malware, spyware, etc., however there are some definite differences between them, which are outlined below.

All computer viruses are Malware, but not all Malware are viruses. Malware encompases a much broader spectrum of MALicious softWARE, of which viruses are just a part of. Malware is covered in more detail in a different section here.

VIRUS: A Virus is a program that has two jobs.

#1. REPLICATE: (Make copies of itself, usually as code hiding in boot sectors or as extra code added to the end of executable programs, and infecting other files.)
#2. ACTIVATE: (When a virus carries out the purpose for which it was designed to do, for example, erase the boot sector of your hard drive.)
A Virus replicates itself to files on the hard drive, or other drives, such as usb drives, flash/thumb drives, and even CD/DVDs. A virus does not replicate across networks, but files already infected with viruses are sent over networks.

WORMS: replicate themselves and spread through networked computers, but unlike viruses, they don't infect other files, instead they keep replicating themselves, the purpose of which is to slow down systems and networks. Worms can, however, carry a payload that contains other viruses or Malware. Some worms damage a system to make it vulnerable to future infection and takeover.

The ultimate function of viruses is to gain control over your system, seeking out information to either steal it, (for example credit card or social security numbers, passwords, banking details), or destroy information, (wiping your hard drive).

Various types of viruses:

* Boot Virus hides on removable media, such as floppy drives, or other removable drives, and attacks your system while it is booting up,
where the files necessary for the computer to boot and run reside. Boot viruses interrupt the computer's ability to boot, rendering the entire PC useless.

* Macro Any type of virus that uses application macros in order to replicate and activate.
For example, Microsoft Excel makes heavy use of macros within the program, as it is designed to do, to make repetitive functions of using the program easy.
A malicious macro will automatically make the application perform functions you don't want to perform.

* Trojan Although they do not replicate, a trojan is Malware that looks or pretends to do one thing, while secretly doing something destructive. A trojan can be disguised as a game, application, or anything to get you to use it. As an example, once you start using it, it can turn your computer into a mini server, opening up your computer ports, exposing it, so a hacker can remotely control your computer secretly. They can also capture your keystrokes, passwords, sensitive information, etc.

* Rootkit A very insidious version of a trojan that burries itself deep in the low level operating system functions, making it nearly impossible to find, and access to all functions of the computer. Only the most advanced and aggressive anti-malware tools will be able to find it. 

* Resident Viruses take up space in your memory (RAM) to hog resources and reduce the performance of your computer, 
infecting and impacting any operation conducted by the system, corrupting files and programs.

* Direct Action Viruses replicate, and execute when activated. This type of virus activates when specific conditions are met and go to work infecting specific file folders and systems, rendering the PC useless in most cases. This type of virus is always located in the root directory and attacks when the computer is booted.

* Overwrite Viruses infects files, deleting or overwriting specific information, rendering the file useless. Files can be "cleaned" by deleting them and reloading or re-creating them.

* Directory viruses are activated when an email attachment is opened with an .exe or .com extension.

* Polymorphic viruses change their encryption every time they are activated, making them invisible to virus scan software, giving them free reign to replicate at will.

Viruses aren't the only threat to your system. Viruses are just a portion of an even GREATER threat, called MalWare, which you can learn about here