While most people lump any kind of malicious code into the virus category, there are some distinctions among the bad guys. A virus is a computer program, or piece of code, that can replicate itself without a users knowledge. A virus is not always malicious, though it is more times than not, and sometimes its mere presence on a system can cause problems. In addition, non-malicious viruses may contain bugs that cause damage. Not all malicious code is a virus. A Trojan horse program, one that comes into your system disguised as something else (and causes damage or compromises security) is not a virus. Internet worms like Klez.H or CodeRed can actually be a combination of threats, and can enter your system through various ways. They can also affect other systems in a multitude of ways. Both worms and Trojans, however, can "drop" viruses into systems. The antivirus software and security vendors refer to these worms as blended threats, and they are currently the focus of much research and development.
Internet worms like Klez.H or CodeRed can actually be a combination of threats, and can enter your system through various ways.
Viruses and malicious code can come in the form of executable programs, document macros, Web page scripts, or even as packets on the Internet never written to disk (as seen in the CodeRed worm). Threats are classified in a number of ways -- operating system (W32,W95, Linux, etc), applications they infect (W97M, WordPro, X97M, etc), type of threat (Worm, Backdoor, Trojan, etc), or language (HTML, VBS, JS, etc). Delivery of malicious codes to a users machine has changed since the first viruses were discovered. Most of recall the most popular early methods of passing viruses by floppy disk. However, the snails-pace speed of infection of such methods have been eclipsed by Internet borne worms that require no human intervention once started.