Virus is a term that is thrown around a lot. The easiest way to describe it is to think about your computer like the human body. A virus is an infection with varying degrees of severity. Some are just mild annoyances, eg. sluggish response, fever (overdriving to system to overheat it), … Some can be fatal, causing actual damage to hardware and software. Malware is an umbrella term for any program that is “bad” for your computer. Short for Malicious Software.
There are also varying degrees of contagiousness. Most viruses are attached to an executable file (exe), but they can spread quite easily by copying themselves into other programs or files as well, even over a network environment or by sending themselves in an infected email.
A Trojan, a type of virus, is malicious software that presents itself as something beneficial in order to trick the user into clicking/installing it. Their effects can be wide ranging, from popups to system corruption. They are also normally the carrier for other malware, especially viruses, bots, back-doors, rootkits, etc.
Back Door Viruses
A Back Door is way for an outside source to access your computer, normally caused by a trojan, virus, or bot. This type of malware is particularly problematic as it hides itself quite well, and leaves you open for additional infections. Any back door is a serious security compromise.
A worm is a form of Malware or Virus that normally propagates through email, instant messaging or documents. Once on the computer, it utilizes security holes to “worm” its way further into your computer or to other computers on your network (home or office), infecting them as well. The thing that differentiates it from a virus is that it is standalone software that does not require a host program or human to help propagate them.
Spyware & Adware type Malware
Spyware is specifically designed to capture information. The most dangerous types of spyware are Keylogger Programs, designed to capture your keystrokes, including usernames and passwords. This information would then be sent to the “designer” of the program.
Adware is software that generates advertisements on your computer. Not all adware is malicious, it can be used in shareware/freeware to generate profit for the programmer rather than charging a price for the license.
Once your computer is infected with malware, it is extremely important for it to avoid detection and removal so that it can continue to run. This is often accomplished through a rootkit, a piece of code designed to “hide” or protect malicious software. Even after removal of the malware, the rootkit can put it back in or make it easier to be infected with other malware.