Definitions:
In computer security technology,
a virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents. A computer virus behaves in a way similar to a biological virus, which spreads by inserting itself into living cells. Extending the analogy, the insertion of a virus into the program is termed as an infection, and the infected file (or executable code that is not part of a file) is called a host. Viruses are one of the several types of malicious software or malware. In a common parlance, the term virus is often extended to refer to worms, trojan horses and other sorts of malware; however, this can confuse computer users, since viruses in the narrow sense of the word are less common than they used to be, compared to other forms of malware. This confusion can have serious consequences, because it may lead to a focus on preventing one genre of malware over another, potentially leaving computers vulnerable to future damage. However, a basic rule is that computer viruses cannot directly damage hardware, but only software.
A computer worm is a self-replicating computer program, similar to a computer virus. A virus attaches itself to, and becomes part of, another executable program; however, a worm is self-contained and does not need to be part of another program to propagate itself. They are often designed to exploit the file transmission capabilities found on many computers. The main difference between a computer virus and a worm is that a virus can not propagate by itself whereas worms can. A worm uses a network to send copies of itself to other systems and it does so without any intervention. In general, worms harm the network and consume bandwidth, whereas viruses infect or corrupt files on a targeted computer. Viruses generally do not affect network performance, as their malicious activities are mostly confined within the target computer itself.
In the context of computer software,
a Trojan horse is a malicious program that is disguised as legitimate software. The term is derived from the classical myth of the Trojan horse. In the siege of Troy, the Greeks left a large wooden horse outside the city. The Trojans were convinced that it was a gift, and moved the horse to a place within the city walls. It turned out that the horse was hollow, containing Greek soldiers who opened the city gates of Troy at night, making it possible for the Greek army to pillage the city. Trojan horse programs work in a similar way: they may look useful or interesting (or at the very least harmless) to an unsuspecting user, but are actually harmful when executed. Trojan horse programs cannot replicate themselves, in contrast to some other types of malware, like viruses or worms. A Trojan horse can be deliberately attached to otherwise useful software by a cracker, or it can be spread by tricking users into believing that it is a useful program.
Malware (a portmanteau of "malicious software") is software designed to infiltrate or damage a computer system, without the owner's consent. The term describes the intent of the creator, rather than any particular features. Malware is commonly taken to include computer viruses, Trojan horses, and spyware. In law, malware is sometimes known as a computer contaminant, for instance in the legal codes of California, Virginia, and several other U.S. states [1]. Malware is sometimes pejoratively called scumware.
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