Viruses, Trojans and Worms

The words Trojan, worm and virus are often used interchangeably, but they are not the same. They are all malicious programs (sometimes called "malware") that can harm your computer, but there are important differences among them.
The commonest computer threats are viruses.
Virus
A virus is a computer program which changes the way in which the computer operates without the knowledge of the user.
A virus must execute itself and it must replicate itself, or make additional copies of itself. Some viruses are a threat to system integrity: they damage computers by altering programs, deleting files or other malicious activities. Others simply announce their presence by audio, video or text messages.
Even viruses which don't do any real damage are a nuisance as they can reduce system performance by wasting memory or slowing down a computer. Most viruses can cause a system to behave oddly and some can cause the system to crash, often making it difficult to restart. Some viruses are a threat to data security, as they attempt to steal confidential data from a machine and transmit it to someone.
Viruses can spread by many methods, including attaching themselves to other files such as programs and documents. Viruses are given different names depending on the way they spread. Some of these different types of virus are explained in later sections.
Simply spreading from computer to computer is not itself a major problem. Most viruses contain a payload, a program that they will execute in addition to spreading themselves. The effects of common virus payloads include deleting or corrupting files or disks and passing on usernames and passwords.