Computer Viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation.
Why people create computer viruses
Computer viruses do not come into existence spontaneously. They are deliberately created by programmers, or by people who use virus creation software.
Virus writers can have various reasons for creating and spreading malware. Viruses have been written as research projects, pranks, vandalism , to attack the products of specific companies, to distribute political messages, and financial gain from identity theft , spyware , and cryptoviral extortion . Some virus writers consider their creations to be works of art, and see virus writing as a creative hobby.
When computer viruses created
A program called " Elk Cloner " is credited with being the first computer virus to appear "in the wild" — that is, outside the single computer or lab where it was created. Written in 1982 by Richard Skrenta , it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk . This virus was originally a joke, created by the high school student and put onto a game. The disk could only be used 49 times.
Ways of computer viruses enter your computer
Viruses are most easily spread by attachments in e-mail messages or instant messaging messages. That is why it is essential that you never open e-mail attachments unless you know who it's from and you are expecting it. Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files. Viruses also spread through downloads on the Internet. They can be hidden in illicit software or other files or programs you might download.
How to protect your computer from computer viruses
Only open e-mail attachments received from sources you trust and that you are expecting.
Back up your hard drive files frequently and maintain multiple backup sets.
If you are experiencing unusually slow performance, apparent file loss or damage, or other abnormalities, immediately check your hard drives and diskettes for infection.
Delete chain emails and junk email. Do not forward or reply to any to them. These types of email are considered spam, which is unsolicited, intrusive mail that clogs up the network.
Download software only from sites you are familiar with, and immediately check it with your anti-virus software.
Don't spread hoaxes and rumors. There are a number of urban legends and other reports of fake email viruses; these cause unneeded panic. If someone sends you an email warning of a virus, please check with the Virus Busters site first before forwarding the message to everyone you know. They maintain a list of known hoaxes, and can save you the embarrassment of panicking everyone on your email address book.
Examples of computer viruses
Trojan Horse: A destructive program that masquerades as a benign application . Unlike viruses , Trojan horses do not replicate themselves but they can be just as destructive. One of the most insidious types of Trojan horse is a program that claims to rid your computer of viruses but instead introduces viruses onto your computer.
Trojan horses are broken down in classification based on how they breach systems and the damage they cause. The seven main types of Trojan horses are:
Remote Access Trojans
Data Sending Trojans
security software disabler Trojans
denial-of-service attack (DoS) Trojans
Worm: A worm is similar to a virus by its design, and is considered to be a sub-class of a virus. Worms spread from computer to computer, but unlike a virus, it has the capability to travel without any help from a person. A worm takes advantage of file or information transport features on your system, which allows it to travel unaided. The biggest danger with a worm is its capability to replicate itself on your system, so rather than your computer sending out a single worm, it could send out hundreds or thousands of copies of itself, creating a huge devastating effect.
Bootsector Virus : A virus which attaches itself to the first part of the hard disk that is read by the computer upon boot up. These are normally spread by floppy disks. Macro Virus: Macro viruses are viruses that use another application's macro programming language to distribute themselves. They infect documents such as MS Word or MS Excel and are typically spread to other similar documents. Memory Resident Viruses: Memory Resident Viruses reside in a computers volitale memory (RAM). They are initiated from a virus which runs on the computer and they stay in memory after it's initiating program closes.
http://www.webopedia.com/DidYouKnow/Internet/2004/virus.asp - The Difference Between a Virus, Worm, and Trojan Horse.