A computer virus is a computer program which reproduces. A computer virus is often simply called a virus. The term is commonly used to refer to a range of malware, but a true virus does not need to be harmful. To distribute itself, a virus needs to execute or otherwise be interpreted. Viruses often hide themselves inside other programs to be executed.

A computer virus reproduces by making, possibly evolved, copies of itself in the computer's memory, storage, or over a network. This is similar to, but not exactly the same as the way a biological virus works. It is estimated by some experts that the Mydoom worm infected a quarter-million computers in a single day in January 2004. Another example is the ILOVEYOU virus, which occurred in 2000 and had a similar effect.

There are tens of thousands of viruses operating in the general Internet today, and new ones are discovered every day. While a generic explanation of how viruses work is difficult due to the wide variety of infection or spreading patterns, there are broad categories commonly used to describe various types of viruses.

Basic types of viruses

Virus types are used as a way for people to think about the things that viruses do, but being overly dogmatic about these types can often be confusing. Take these descriptions for the value they bring, but don't assume that they are comprehensive or particularly accurate in terms of describing the potential sorts of viruses that can be encountered.

Network viruses

This kind of virus is proficient in quickly spreading across a Local Area Network (LAN) or even over the Internet. Usually, it propagates through shared resources, such as shared drives and folders. Once it infects a new system, it searches for potential targets by searching the network for other vulnerable systems. Once a new vulnerable system is found, the network virus infects the other system, and thus spreads over the network. Some of the most notorious network viruses are Nimda and SQLSlammer.

File-Destructive Trojans

The only function of these Trojans is to destroy and delete files. They can automatically delete all the core system files on your machine. The Trojan could be controlled by the attacker or could be programmed to strike like a logic bomb, starting on a specific day or hour.

The main idea behind Denial of Service (DoS) Attack Trojans is to generate a lot of Internet traffic on the victim’s machine, to the extent that the Internet connection is too overloaded to let the user visit a website or download anything. Another variation of a DoS Trojan is the mail-bomb Trojan, whose main aim is to infect as many machines as possible and simultaneously attack specific email addresses with random subjects and contents that cannot be filtered.

from : en.wikipedia.org