“Virus” is a term used to describe malicious programs that unwantedly install themselves into the computer.
Viruses will cause a range of damage, from the mild to the entire loss of your computer data.
A way to describe viruses is to call them “malware”.
Viruses/malware are commonly broken down into Classic Viruses, Trojans, Worms, adware, and spyware.
Classic viruses are programs that rewrite existing computer code on your computer. Classic viruses are not so much unwanted additions to your system as they are mutations of existing code.
Trojans, or Trojan Horses, are additions to your system. These malicious programs masquerade as legitimate files in your email, deceiving you into willfully adding them to your hard drive. Trojans rely on you to intentionally open your computer to them. Once on your machine, Trojans then function as independent programs that operate secretly. Commonly, Trojans steal passwords or perform “denial of service” (overload your system) attacks. Examples of trojans include Backdoor and Nuker.
Worms, or Internet Worms, are also unwanted additions to your system. Worms are different from Trojans, though, because they copy themselves without your direct assistance… they robotically worm their way into your email, and begin broadcasting copies of themselves without permission.
Because they do not require user intervention to reproduce, worms reproduce at an alarming rate. Examples of worms include Scalper, SoBig, and Swen.
Adware and Spyware are cousins to trojans, worms, and viruses. These programs “lurk” on your machine. Adware and spyware are designed to observe your Internet habits and then pummel you with advertising, or to report back to their owners via secret messages.